Etymology
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thick (adj.)
Old English þicce "dense, viscous, solid, stiff; numerous, abundant; deep," also as an adverb, "thickly, closely, often, frequently," from Proto-Germanic *thiku- (source also of Old Saxon thikki, Old High German dicchi, German dick, Old Norse þykkr, Old Frisian thikke), from PIE *tegu- "thick" (source also of Gaelic tiugh). Secondary Old English sense of "close together" is preserved in thickset and proverbial phrase thick as thieves (1833). Meaning "stupid" is first recorded 1590s. Related: Thickly.

As a noun, "the thick part" (of anything), from mid-13c. Phrase through thick and thin, indicating rough or smooth going, hence "unwaveringly," is in Chaucer (late 14c.); thick-skinned is attested from 1540s; in figurative sense from c. 1600. To be in the thick of some action, etc., "to be at the most intense moment" is from 1680s, from a Middle English noun sense.
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thickness (n.)
Old English þicness "density, viscosity, hardness; depth; anything thick or heavy; darkness; thicket;" see thick + -ness.
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thickset (adj.)
also thick-set, late 14c., thikke sette "with parts or things set close together" (of grass on a sward, etc.), from thick + set (v.). Meaning "stocky, strong and square-built" is recorded from 1724.
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thicken (v.)
late 14c. (transitive), 1590s (intransitive), from thick + -en (1). Related: Thickened; thickening. An earlier verb was Middle English thick, Old English þiccian "to thicken, to crowd together."
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thicket (n.)
"close-set growth of shrubs, bushes, trees, etc.," late Old English þiccet, from þicce (see thick) + denominative suffix -et. Absent in Middle English, reappearing early 16c., perhaps a dialectal survival or a re-formation.
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pachycephalic (adj.)

in zoology, "thick-headed," by 1862, from pachy- "thick, large" + -cephalic. Related: Pachycephalous (1890).

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pachy- 

word-forming element in science meaning "thick, large, massive," from Latinized form of Greek pakhys "thick, fat, well-fed, dense, stout,"  from PIE *bhengh- "thick, fat" (source also of Sanskrit bahu- "much, numerous;" Avestan bazah- "height, depth;" Armenian bazum "much;" Hittite pankush "large," panku- (adj.) "total;" Old Norse bingr "heap," Old High German bungo "a bulb;" Latvian biezs "thick").

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mush (n.)

"kind of porridge; meal boiled in water or milk until it forms a thick, soft mass," 1670s, in the American colonies, a variant of mash (n.) "soft mixture." Meaning "anything soft and thick" is attested from 1824.

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inspissate (v.)
"make thick or thicker," 1620s, from Late Latin inspissatus, past participle of inspissare, from in- "in, into" (from PIE root *en "in") + Latin spissare "to thicken," related to spissus "thick" (see spissitude). Related: Inspissated; inspissating.
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loden (n.)
"coarse woolen cloth," 1880, from German loden "thick woolen cloth."
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