thunk (n.)

Old English þencan "imagine, conceive in the mind; consider, meditate, remember; intend, wish, desire" (past tense þohte, past participle geþoht), probably originally "cause to appear to oneself," from Proto-Germanic *thankjan (source also of Old Frisian thinka, Old Saxon thenkian, Old High German denchen, German denken, Old Norse þekkja, Gothic þagkjan).

Old English þencan is the causative form of the distinct Old English verb þyncan "to seem, to appear" (past tense þuhte, past participle geþuht), from Proto-Germanic *thunkjan (source also of German dünken, däuchte). Both are from PIE *tong- "to think, feel" which also is the root of thought and thank.

The two Old English words converged in Middle English and þyncan "to seem" was absorbed, except for its preservation in archaic methinks "it seems to me."

As a noun, think, "act of prolonged thinking," is attested by 1834. The figurative thinking cap is attested from 1839.

Old English drincan "to swallow water or other fluid," also "to swallow up, engulf" (class III strong verb; past tense dranc, past participle druncen), from Proto-Germanic *drenkanan (source also of Old Saxon drinkan, Old Frisian drinka, Dutch drinken, Old High German trinkan, German trinken, Old Norse drekka, Gothic drigkan "to drink"), which is of uncertain origin or connections, perhaps from a root meaning "to draw."

Most Indo-European words for this trace to PIE *po(i)- (source of Greek pino, Latin biber, Irish ibim, Old Church Slavonic piti, Russian pit'; see imbibe).

Figurative meaning "take in through the senses" is from late 12c. Especially "to imbibe spiritous liquors" from mid-15c. To drink to "salute in drinking" is by mid-13c. To drink like a fish is recorded from 1744. To drink (someone) under the table "continue drinking and remain (comparatively) sober after others have passed out" is by 1909.

Middle English sinken, from Old English sincan (intransitive) "become submerged, go under, subside" (past tense sanc, past participle suncen), from Proto-Germanic *senkwan (source also of Old Saxon sinkan, Old Norse sökkva, Middle Dutch sinken, Dutch zinken, Old High German sinkan, German sinken, Gothic sigqan), from PIE root *sengw- "to sink."

The transitive use "force or drag gradually downward" (attested from late 12c.) supplanted Middle English sench (compare drink/drench) which died out 14c. The sense of "go in, penetrate" (of a blow, a weapon, etc.) is from c. 1300; by early 14c. as "make a penetrating impression on the mind." Related: Sank; sunk; sinking.

From early 14c. as "be reduced to a lower or worse state;" late 14c. as "fall or fail as from weakness or under a heavy blow." From 1590s as "decrease, be reduced in value, amount, etc." Of the sun, moon, etc., "to set," c. 1600. Of land, "dip downward gradually," by 1726.

Contrasted with swim (v.) since 14c.; the adjectival phrase sink or swim is from 1660s. To sink without a trace is World War I military jargon, translating German spurlos versenkt.