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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able-bodied (adj.)
"healthy and sufficiently strong," 1620s; see able + body.
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workhouse (n.)
Old English weorchus "workshop;" see work (n.) + house (n.). From 1650s in the sense of "place where the able-bodied poor or petty criminals are lodged and compelled to work."
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flatworm (n.)

name applied to animals of the planarian group, a tapeworm or other simple unsegmented, soft-bodied invertebrate, 1721; see flat (adj.) + worm (n.). So called for their shape.

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mollusk (n.)

"soft-bodied invertebrate animal, usually with an external shell," 1783, mollusque (modern spelling from 1839), from French mollusque, from Modern Latin Mollusca (see Mollusca), the phylum name. Related: Molluscuous; molluscan.

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pachycephalic (adj.)

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

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stout (adj.)

c. 1300, "proud, valiant, strong," from Old French estout "brave, fierce, proud," earlier estolt "strong," from a Germanic source from West Germanic *stult- "proud, stately, strutting" (source also of Middle Low German stolt "stately, proud," German stolz "proud, haughty, arrogant, stately"), from PIE root *stel- "to put, stand, put in order," with derivatives referring to a standing object or place.

Meaning "strong in body, powerfully built" is attested from late 14c., but has been displaced by the (often euphemistic) meaning "thick-bodied, fat and large, bulky in figure," which is first recorded 1804. Original sense preserved in figurative phrase stout-hearted (1550s). Related: Stoutly; stoutness.

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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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A.B. 

affixed to a name, abbreviation of Modern Latin Artium Baccalaureus "Bachelor of Arts" (see bachelor), 1773, American English. British English preferred B.A., perhaps because A.B. was used in Britain to mean able-bodied on seamen's papers.

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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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