thankyoudaoicibaDictYouDict[thank 词源字典]
thank: [OE] The notion of ‘gratitude’ in modern English thank arose out of an earlier ‘thoughtfulness’. For the word goes back ultimately to prehistoric Germanic *thank-, *thengk-, which also produced English think, and the noun thank originally meant ‘thought’ (a 12th-century translation of the gospels has ‘From the heart come evil thanks’ Matthew 15:19, where the Authorized Version gives ‘Out of the heart proceed evil thoughts’).

The sense ‘thought’ graduated via ‘favourable thought, good will’ to ‘gratitude’. It was originally singular, and the modern plural usage did not emerge until the 14th century. Thank you first appeared in the 15th century, short for I thank you.

=> think[thank etymology, thank origin, 英语词源]
thank (v.)youdaoicibaDictYouDict
Old English þancian, þoncian "to give thanks, thank, to recompense, reward," from Proto-Germanic *thankojan (cognates: Old Saxon thancon, Old Norse þakka, Danish takke, Old Frisian thankia, Old High German danchon, Middle Dutch, Dutch, German danken "to thank"), from *thankoz "thought, gratitude," from PIE root *tong- "to think, feel."

Related phonetically to think as song is to sing; for sense evolution, compare Old High German minna "loving memory," originally "memory." Related to Old English noun þanc, þonc, originally "thought," but by c. 1000 "good thoughts, gratitude." In ironical use, "to blame," from 1550s. To thank (someone) for nothing is recorded from 1703. Related: Thanked; thanking.