is the causative form of the distinct O.E. verb þyncan "to seem or appear" (past tense þuhte, pp. geþuht), from P.Gmc. *thunkjan (cf. Ger. dünken, däuchte). Both are from PIE *tong- "to think, feel" which also is the root of thought
. The two meanings converged in M.E. and þyncan "to seem" was absorbed, except for archaic methinks
"it seems to me." Jocular pp. thunk (not historical, but by analogy of drink
, etc.) is recorded from 1876. Think-tank is 1959 as "research institute" (first ref. is to Center for Behavioral Sciences, Palo Alto, Calif.); it had been colloquial for "the brain" since 1905.