Etymology
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*legh- 
Proto-Indo-European root meaning "to lie down, lay."

It forms all or part of: allay; anlage; belay; beleaguer; bylaw; coverlet; fellow; lager; lair; law; lawful; lawless; lawsuit; lawyer; lay (v.) "to cause to lie or rest;" ledge; ledger; lees; lie (v.2) "rest horizontally;" litter; lochia; low (adj.) "not high;" outlaw; scofflaw; stalag; vorlage.

It is the hypothetical source of/evidence for its existence is provided by: Hittite laggari "falls, lies;" Greek lekhesthai "to lie down," legos "bed," lokhos "lying in wait, ambush," alokhos "bedfellow, wife;" Latin lectus "bed;" Old Church Slavonic lego "to lie down;" Lithuanian at-lagai "fallow land;" Old Irish laigim "I lie down," Irish luighe "couch, grave;" Old English licgan "be situated, have a specific position; remain; be at rest, lie down."
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*kad- 
Proto-Indo-European root meaning "to fall."

It forms all or part of: accident; cadaver; cadence; caducous; cascade; case (n.1); casual; casualty; casuist; casus belli; chance; cheat; chute (n.1); coincide; decadence; decay; deciduous; escheat; incident; occasion; occident; recidivist.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sad- "to fall down;" Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," cadere "to fall, sink, settle down, decline, perish;" Armenian chacnum "to fall, become low;" perhaps also Middle Irish casar "hail, lightning."
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*me- (4)

*mē-, Proto-Indo-European root meaning "to cut down grass or grain." It forms all or part of: aftermath; math (n.2) "a mowing;" mead (n.2) "meadow;" meadow; mow (v.).

It is the hypothetical source of/evidence for its existence is provided by: Greek (poetic) amao, Latin metere "to reap, mow, crop;" Italian mietere, Old Irish meithleorai "reapers," Welsh medi; Old English mawan "to mow," mæd "meadow."

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*pau- (2)

Proto-Indo-European root meaning "to cut, strike, stamp."

It forms all or part of: account; amputate; amputation; anapest; berate; compute; count (v.); depute; deputy; dispute; impute; pave; pavement; pit (n.1) "hole, cavity;" putative; rate (v.1) "to scold;" reputation; repute.

It is the hypothetical source of/evidence for its existence is provided by: Latin pavire "to beat, ram, tread down," putare "to prune;" Greek paiein "to strike;" Lithuanian pjauti "to cut," pjūklas "saw."

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*dheigw- 

Proto-Indo-European root meaning "to stick, fix." 

It forms all or part of: affix; crucifix; crucify; dig; dike; ditch; fibula; fiche; fichu; fix; fixate; fixation; fixity; fixture; infibulate; infibulation; microfiche; prefix; suffix; transfix.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dehi- "wall;" Old Persian dida "wall, stronghold, fortress," Persian diz; Latin figere "to fix, fasten, drive, thrust in; pierce through, transfix;" Lithuanian dygstu, dygti "germinate;" Old Irish dingid "presses, thrusts down;" Old English dic "trench, ditch," Dutch dijk "dam." 

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*leubh- 

Proto-Indo-European root meaning "to care, desire, love."

It forms all or part of: belief; believe; furlough; leave (n.) "permission, liberty granted to do something;" leman; libido; lief; livelong; love; lovely; quodlibet.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit lubhyati "desires," lobhaya- "to make crazy;" Persian ahiftan "to be tangled, be hit down, be in love;" Latin lubet, later libet "pleases," libido, lubido "desire, longing; sensual passion, lust;" Old Church Slavonic l'ubu "dear, beloved," ljubiti, Russian ljubit' "to love;" Lithuanian liaupsė "song of praise;" Old English lufu "feeling of love; romantic sexual attraction," German Liebe "love," Gothic liufs "dear, beloved."

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*klei- 

Proto-Indo-European root meaning "to lean."

It forms all or part of: acclivity; anticline; clemency; client; climate; climax; cline; clinic; clinical; clino-; clitellum; clitoris; decline; declivity; enclitic; heteroclite; incline; ladder; lean (v.); lid; low (n.2) "small hill, eminence;" matroclinous; patroclinous; polyclinic; proclitic; proclivity; recline; synclinal; thermocline.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian šlyti "to slope," šlieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting").

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*kae-id- 

*kaə-id-, Proto-Indo-European root meaning "to strike."

It forms all or part of: abscise; avicide; biocide; caesarian; caesura; cement; chisel; -cide; circumcise; circumcision; concise; decide; decision; deicide; excise (v.); excision; felicide; feticide; filicide; floricide; fratricide; fungicide; gallinicide; genocide; germicide; herbicide; homicide; incise; incision; incisor; infanticide; insecticide; legicide; liberticide; libricide; matricide; parricide; patricide; pesticide; precise; precision; prolicide; scissors; senicide; spermicide; suicide; uxoricide; verbicide.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit skhidati "beats, tears;" Latin caedere "to strike down, fell, slay;" Lithuanian kaišti "shave;" Armenian xait'em "to stab;" Albanian qeth "to shave;" Middle Dutch heien "to drive piles," Old High German heia "wooden hammer," German heien "beat."

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*pet- 

Also petə-, Proto-Indo-European root meaning "to rush, to fly." 

It forms all or part of: accipiter; appetence; appetite; apterous; apteryx; archaeopteryx; asymptote; centripetal; Coleoptera; compete; competent; eurypterid; feather; helicopter; hippopotamus; Hymenoptera; impetigo; impetuous; impetus; iopterous; Lepidoptera; ornithopter; panache; panne; pen (n.1) "writing implement;" pennon; peripeteia; perpetual; perpetuity; petition; petulance; petulant; pin; pinion; pinnacle; pinnate; pinniped; potamo-; potamology; propitiation; propitious; ptero-; pterodactyl; ptomaine; ptosis; repeat; symptom.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pattram "wing, feather, leaf," patara- "flying, fleeting;" Hittite pittar "wing;" Greek piptein "to fall," potamos "river, rushing water," pteron, pteryx "feather, wing," ptilon "soft feathers, down, plume;" Latin petere "to attack, assail; seek, strive after; ask for, beg; demand, require," penna "feather, wing;" Old Norse fjöðr, Old English feðer "feather;" Old Church Slavonic pero "feather;" Old Welsh eterin "bird."

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*stere- 
*sterə-, also *ster-, Proto-Indo-European root meaning "to spread."

It forms all or part of: consternate; consternation; construct; construction; destroy; destruction; industry; instruct; instruction; instrument; obstruct; obstruction; perestroika; prostrate; sternum; sternocleidomastoid; strain (n.2) "race, stock, line;" stratagem; strategy; strath; strato-; stratocracy; stratography; stratosphere; stratum; stratus; straw; stray; street; strew; stroma; structure; substrate; substratum; substructure.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit strnoti "strews, throws down;" Avestan star- "to spread out, stretch out;" Greek stronymi "strew," stroma "bedding, mattress," sternon "breast, breastbone;" Latin sternere "to stretch, extend;" Old Church Slavonic stira, streti "spread," strana "area, region, country;" Russian stroji "order;" Gothic straujan, Old High German strouwen, Old English streowian "to sprinkle, strew;" Old English streon "strain," streaw "straw, that which is scattered;" Old High German stirna "forehead," strala "arrow, lightning bolt;" Old Irish fo-sernaim "spread out," srath "a wide river valley;" Welsh srat "plain."
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