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.
"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.
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.
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.