Thank

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Old English thancian, of Germanic origin; related to Dutch and German danken ; compare with thanks.

wiktionary

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From Middle English thank, from Old English þanc(“thought, favour, grace, pleasure, satisfaction, thanks”), from Proto-Germanic *þankaz(“thought, remembrance, gratitude”), from Proto-Indo-European *tong-, *teng-(“to think”). Cognate with Saterland Frisian Tonk, West Frisian tank, Dutch dank, Low German Dank, German Dank, Danish tak, Swedish tack, Faroese tøkk, Icelandic þökk. Related to thought.

From Middle English thanken, thankien, from Old English þancian, þoncian(“to thank, give thanks”), from Proto-Germanic *þankōną(“to thank”), from Proto-Germanic *þankaz(“thought, gratitude”), from Proto-Indo-European *teng-(“to think, feel”). Cognate with Saterland Frisian tonkje(“to thank”), West Frisian tanke(“to thank”), Dutch danken(“to thank”), Low German danken(“to thank”), German danken(“to thank”), Danish takke(“to thank”), Swedish tacka(“to thank”), Icelandic þakka(“to thank”). Related to thought.

etymonline

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thank (v.)

Old English þancian, þoncian "to give thanks, thank, to recompense, to reward," from Proto-Germanic *thankōjanan (source also of 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."

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