O.E.
þancian "to give thanks," from P.Gmc.
*thankojan (cf. O.S.
thancon, O.N.
þakka, Dan.
takke, O.Fris.
thankia, M.Du., Ger.
danken "to thank"), from
*thankoz "thought, gratitude," from PIE base
*tong- "to think, feel." For sense evolution, cf. related O.E. noun
þanc, þonc, originally "thought," but by c.1000 "good thoughts, gratitude." The whole group is from the same root as
think (q.v.). In ironical use, "to blame," from 1560.
Thankful is from O.E.
þancfulle; thankless "likely to not be rewarded with thanks" is from c.1547.
Thank you is attested from c.1400, short for
I thank you. To
thank (someone) for nothing is recorded from 1703.