Etymology
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*spek- 
Proto-Indo-European root meaning "to observe."

It forms all or part of: aspect; auspex; auspices; auspicious; bishop; circumspect; conspicuous; despicable; despise; episcopal; especial; espionage; espy; expect; frontispiece; gyroscope; haruspex; horoscope; inspect; inspection; inspector; introspect; introspection; perspective; perspicacious; perspicacity; prospect; prospective; respect; respite; retrospect; scope; -scope; scopophilia; -scopy; skeptic; species; specimen; specious; spectacle; spectacular; spectrum; speculate; speculation; speculum; spice; spy; suspect; suspicion; suspicious; telescope.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit spasati "sees;" Avestan spasyeiti "spies;" Greek skopein "behold, look, consider," skeptesthai "to look at," skopos "watcher, one who watches;" Latin specere "to look at;" Old High German spehhon "to spy," German spähen "to spy."
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*mendh- 

Proto-Indo-European root meaning "to learn." It forms all or part of: chrestomathy; mathematic; mathematical; mathematics; opsimathy; polymath.

It is the hypothetical source of/evidence for its existence is provided by: Greek menthere "to care," manthanein "to learn," mathēma "science, knowledge, mathematical knowledge;" Lithuanian mandras "wide-awake;" Old Church Slavonic madru "wise, sage;" Gothic mundonsis "to look at," German munter "awake, lively."

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*keu- 

also *skeu- Proto-Indo-European root meaning "to see, observe, perceive." 

It forms all or part of: Anschauung; caution; cautious; caveat; kudos; precaution; scavenger; scone; sheen; show.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kavih "wise, sage; seer, poet;" Avestan kauui- "seer, poet, wise man;" Middle Iranian škoh "splendor, majesty;" Latin cautio "care, foresight," cautus "careful, heedful," cavere "beware, take heed;" Greek kydos "glory, fame;" Lithuanian kavoti "tend, safeguard;" Armenian cucanem "I show;" Old Church Slavonic čudo "wonder;" Czech (z)koumati "to perceive, be aware of;" Serbian čuvati "watch, heed;" Old English sceawian "to look at," Middle Dutch schoon "beautiful, bright," properly "showy," Old High German scouwon "to watch."

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*g(a)lag- 
also *g(a)lakt-, Proto-Indo-European root meaning "milk."

It forms all or part of: ablactation; cafe au lait; galactic; galaxy; lactate (v.); lactate (n.); lactation; lacteal; lactescence; lactic; lactivorous; lacto-; lactose; latte; lettuce.

It is the hypothetical source of/evidence for its existence is provided by: Latin lac (genitive lactis) "milk;" Greek gala (genitive galaktos), "milk;" Armenian dialectal kaxc' "milk." The initial "g" probably was lost in Latin by dissimilation. This and the separate root *melg-, account for words for "milk" in most of the Indo-European languages. The absence of a common word for it is considered a mystery.
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cannibalise (v.)
chiefly British English spelling of cannibalize (q.v.); for suffix, see -ize. Related: Cannibalised; cannibalising; cannibalisation.
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*em- 

Proto-Indo-European root meaning "to take, distribute." 

It forms all or part of: assume; consume; emption; example; exemplar; exemplary; exemplify; exempt; exemption; impromptu; peremptory; pre-emption; premium; presume; presumption; prompt; pronto; ransom; redeem; redemption; resume; sample; sejm; subsume; sumptuary; sumptuous; vintage.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit yamati "holds, subdues;" Latin emere "buy," originally "take," sumere "to take, obtain, buy;" Old Church Slavonic imo "to take;" Lithuanian imu, imti "to take."

For the sense shift from "take" to "buy" in the Latin verbs, compare Old English sellan "to give," source of Modern English sell "to give in exchange for money;" Hebrew laqah "he bought," originally "he took;" and colloquial English I'll take it for "I'll buy it." 

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*pel- (4)

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

It forms all or part of: bibliopole; monopolize; monopoly; oligopolistic; oligopoly.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit panate "barters, purchases;" Lithuanian pelnas "gain;" Greek pōlein "to sell;" Old Church Slavonic splenu, Russian polon "prey, booty;" Old Norse falr, Dutch veil, German feil "for sale, venal."  

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*wer- (3)

Proto-Indo-European root meaning "perceive, watch out for."

It forms all or part of: Arcturus; avant-garde; award; aware; beware; Edward; ephor; garderobe; guard; hardware; irreverence; lord; panorama; pylorus; rearward; regard; revere; reverence; reverend; reward; software; steward; vanguard; ward; warden; warder; wardrobe; ware (n.) "manufactured goods, goods for sale;" ware (v.) "to take heed of, beware;" warehouse; wary.

It is the hypothetical source of/evidence for its existence is provided by: Latin vereri "to observe with awe, revere, respect, fear;" Greek ouros "a guard, watchman," horan "to see;" Hittite werite- "to see;" Old English weard "a guarding, protection; watchman, sentry, keeper."

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*ned- 
Proto-Indo-European root meaning "to bind, tie."

It forms all or part of: annex; annexation; connect; connection; denouement; net (n.) "netting, network, mesh used for capturing;" nettle; nexus; node; nodule; noose.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nahyati "binds, ties;" Latin nodus "knot;" Old Irish nascim "I bind, oblige;" Old English net "netting, network."
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*stegh- 
Proto-Indo-European root meaning "to stick, prick, sting." It forms all or part of: stag; sting; stochastic.

It is the hypothetical source of/evidence for its existence is provided by: Greek stokhos "fixed target, erected pillar for archers to shoot at;" Lithuanian stagaras "long, thin stalk of a plant;" Old English stagga "stag," stingan "to sting;" Old Danish stag "point;" Old Norse stong "stick, pole."
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