Origins
The English verb 'think' descends from Old English 'þencan' (to think, consider, intend, conceive in the mind), from Proto-Germanic *þankijaną, from the PIE root *tong- meaning 'to think' or 'to feel.' The word has been a cornerstone of English expression since the earliest recorded texts, and its family of derivatives — including 'thought,' 'thank,' and the archaic 'methinks' — illuminates the deep connections between cognition, gratitude, and perception in the Germanic worldview.
The Proto-Germanic root *þank- produced two closely related verb families in Old English that are often confused. Old English 'þencan' (to think, to consider) was a weak verb (Class I) with the past tense 'þōhte' — ancestor of Modern English 'thought.' This is the direct ancestor of 'think.' A separate but related verb, Old English 'þyncan' (to seem, to appear), was an impersonal verb that took a dative subject: 'mē þyncþ' meant 'it seems to me.' This verb survives only in the archaic expression 'methinks' (literally 'it seems to me'), famously used by Shakespeare and now employed only as a conscious archaism.
The merger of 'þencan' and 'þyncan' was already underway in late Old English and was essentially complete by the end of the Middle English period. The two verbs were phonologically similar and semantically adjacent — what one thinks and what seems to one are closely related experiences — and their paradigms gradually collapsed into a single verb.
Germanic Development
The relationship between 'think' and 'thank' is one of the most illuminating in English etymology. Both derive from Proto-Germanic *þank-, and in Old English, 'þancian' (to thank) meant literally 'to think favorably of,' 'to express grateful thought toward.' A thank is, at its etymological core, a kind thought directed at another person. This connection is transparent in German, where 'denken' (to think) and 'danken' (to thank) are obviously related, and the noun 'Gedanke' (thought) is clearly cognate with 'Dank' (thanks).
The past tense 'thought' (Old English 'þōht,' Middle English 'thought') shows an irregular formation typical of the weak verb class to which 'think' belongs. The dental suffix *-t- that marks Germanic weak past tenses combined with the root vowel to produce a form that looks highly irregular by modern standards: think/thought rather than the expected *thinked. This same pattern appears in 'bring/brought,' 'buy/bought,' 'seek/sought,' 'teach/taught,' 'catch/caught,' and 'work/wrought.' These verbs are sometimes called 'rückumlaut' verbs by specialists, because the vowel change in the past tense reverses the i-umlaut that affected the present tense.
Outside Germanic, the PIE root *tong- is not richly attested, and some specialists question whether the root should be reconstructed at all, viewing the 'think/thank' family as a purely Germanic development. However, Latin 'tongēre' (to know, to think) has been proposed as a cognate, though this form is rare and its connection is not universally accepted.
Modern Usage
In Modern English, 'think' participates in a subtle semantic distinction that many languages encode with separate verbs. 'I think' can express opinion ('I think it's going to rain'), active cognition ('I was thinking about the problem'), intention ('I think I'll go home'), or memory ('I was thinking about the old days'). Some languages, like Spanish, distinguish between rational belief ('creer') and cogitative activity ('pensar'), while English uses 'think' for both.
The word has also developed important grammatical functions. 'I think' frequently serves as a hedging device or epistemic marker ('this is, I think, the right answer'), weakening rather than strengthening the speaker's commitment to a claim. Linguists have noted that this parenthetical use of 'I think' has become so grammaticalized that it functions more like an adverb meaning 'probably' than like a full verb of cognition.
Compounds and derivatives include 'thinker' (one who thinks — and by extension, a philosopher or intellectual), 'rethink' (to think again), 'unthinkable' (beyond the scope of thought), 'overthink' (to think excessively), 'freethinking' (thinking unconstrained by dogma), and 'groupthink' (conformist thinking within a group, coined by William H. Whyte in 1952 and popularized by Irving Janis in 1972).