Etymology
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Words related to thought

think (v.)

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.

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afterthought (n.)
1660s, "a later thought," from after + thought (n.). As "reflection after an act," 1680s. Sense of "youngest child of a family" (especially one born much later than the others) is by 1902.
brought 
past tense and past participle of bring (v.). For explanation of the form development, see thought.
merrythought (n.)

"wishbone of a fowl's breast," c. 1600, from merry (adj.) + thought. So called from the sport of breaking it between two persons pulling each on an end to determine who will get a wish he made for the occasion (the winner getting the longer fragment). Also see wishbone.

thoughtful (adj.)
c. 1200, "contemplative, occupied with thought," from thought + -ful. Also in Middle English, "prudent; moody, anxious." Meaning "showing consideration for others" is from 1851 (compare thoughtless.) Related: Thoughtfully; thoughtfulness.
thoughtless (adj.)
1610s, "heedless, imprudent," from thought + -less. Meaning "inconsiderate of others" is from 1794. Related: Thoughtlessly; thoughtlessness.
tooth (n.)

Old English toð (plural teð), from Proto-Germanic *tanthu- (source also of Old Saxon, Danish, Swedish, Dutch tand, Old Norse tönn, Old Frisian toth, Old High German zand, German Zahn, Gothic tunþus), from PIE root *dent- "tooth." Plural teeth is an instance of i-mutation.

The loss of -n- before spirants is regular in Old English, Old Frisian, and Old Saxon: compare goose (n.), five, mouth (n.). Also thought, from stem of think; couth from the stem of can (v.1); us from *uns.

Application to tooth-like parts of other objects (saws, combs, etc.) first recorded 1520s. Tooth and nail as weapons is from 1530s. The tooth-fairy is attested from 1964.