Old English þicce "not thin, dense," from Proto-Germanic *theku-, *thekwia- (cf. Old Saxon thikki, Old High German dicchi, German dick, Old Norse þykkr, Old Frisian thikke), from PIE *tegu- "thick" (cf. 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. Phrase thick and thin 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.
adj. thick·er, thick·est
Relatively great in extent from one surface to the opposite, usually in the smallest solid dimension; not thin.
Measuring a specified number of units in this dimension.
Heavy in form, build, or stature; thickset.
Having component parts in a close, crowded state or arrangement; dense.
Having or suggesting a heavy or viscous consistency.
Having a great number; abounding.
Impenetrable by the eyes.
Not easy to hear or understand; indistinctly articulated.
Noticeably affecting sound; conspicuous.
Producing indistinctly articulated sounds.
In a close, compact state or arrangement; densely.
In a thick manner; deeply or heavily.
To stay with someone or something “through thick and thin” is to persevere through good times as well as bad: “She stood beside her friend through thick and thin.”
Under no conceivable circumstances: There's no way I'll ever see it your way (1975+)