Etymology
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flank (v.)
1590s (military), "to guard the flank," also, "to menace the flank, fire sideways upon," from flank (n.). Meaning "stand or be placed at the side of" is from 1650s. Related: Flanked; flanking.
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flank (n.)
late Old English flanc "flank, fleshy part of the side," from Old French flanc "hip, side," from Frankish or another Germanic source, from Proto-Germanic *hlanca- (source also of Old High German (h)lanca, Middle High German lanke "hip joint," German lenken "to bend, turn aside;" Old English hlanc "loose and empty, slender, flaccid;" Old Norse hlykkr "a bend, noose, loop"), from PIE root *kleng- "to bend, turn" (see link (n.)). Showing characteristic change of Germanic hl- to Romanic fl-. The military sense is first attested 1540s. Meaning "side" of anything is by 1620s. As an adjective, "pertaining to the flank or side," 1660s. Related: Flanked; flanking.
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fianchetto (n.)
chess move, 1847, Italian, diminutive of fianco "flank (attack)," from Old French flanc "hip, side" (see flank (n.)).
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outflank (v.)

1765, "to extend or get beyond the flank" (of an opposing army), from out- + flank (v.). Figurative use, get the better of, outmaneuver," is from 1773. Related: Outflanked; outflanking.

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flange (n.)
1680s, "a widening or branching out," of unknown origin, perhaps related to Old French flanche "flank, hip, side," fem. of flanc (see flank (n.)). Meaning "projecting rim, etc., used for strength or guidance" is from 1735. As a verb from 1820.
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lank (adj.)

Old English hlanc "loose and empty, meagerly slim, flaccid," from Proto-Germanic *hlanka-, forming words meaning "to bend, turn," perhaps from PIE root *kleng- "to bend, turn," with a connecting notion of "flexible" (compare German lenken "to bend, turn aside;" see flank (n.)). In Middle English, "Some examples may be long adj. with unvoicing of g" [The Middle English Compendium]. In reference to hair, "straight and flat," from 1680s. Related: Lankness (1640s).

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link (n.)
early 15c., "one of a series of rings or loops which form a chain; section of a cord," probably from Old Norse *hlenkr or a similar Scandinavian source (compare Old Norse hlekkr "link," in plural, "chain;" Old Swedish lænker "chain, link," Norwegian lenke "a link," Danish lænke "a chain," German Gelenk "articulation, a joint of the body; a link, ring"), from Proto-Germanic *khlink- (source also of German lenken "to bend, turn, lead"), from PIE root *kleng- "to bend, turn." Related to lank, flank, flinch.

The noun is not found in Old English, where it is represented by lank "the hip" ("turn of the body"), hlencan (plural) "armor." Meaning "a division of a sausage made in a continuous chain" is from mid-15c. Meaning "anything serving to connect one thing or part with another" is from 1540s. Sense of "means of telecommunication between two points" is from 1911. Missing link between man and apes dates to 1880.
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ilium (n.)
pelvic bone, 1706, Modern Latin, from Latin ilia (plural) "groin, flank, side of the body from the hips to the groin" (see ileum). In Middle English it meant "lower part of the small intestine." Vesalius gave the name os ilium to the "bone of the flank."
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pleura (n.)

"sereus membrane lining the chest cavity," early 15c., from medical Latin, from Greek pleuron "a rib," in plural, pleura, "the ribs, side of the body," also "flank of an army, page of a book," a word of unknown origin.

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