The Collaborative International Dictionary of English v.0.48:
thank \thank\ (th[a^][ng]k), n.; pl. thanks (th[a^][ng]ks).
[AS. [thorn]anc, [thorn]onc, thanks, favor, thought; akin to
OS. thank favor, pleasure, thanks, D. & G. dank thanks, Icel.
[thorn]["o]kk, Dan. tak, Sw. tack, Goth. [thorn]agks thanks;
-- originally, a thought, a thinking. See Think.]
A expression of gratitude; an acknowledgment expressive of a
sense of favor or kindness received; obligation, claim, or
desert, or gratitude; -- now generally used in the plural.
"This ceremonial thanks." --Massinger.
If ye do good to them which do good to you, what thank
have ye? for sinners also do even the same. --Luke vi.
What great thank, then, if any man, reputed wise and
constant, will neither do, nor permit others under his
charge to do, that which he approves not, especially in
matter of sin? --Milton.
Thanks, thanks to thee, most worthy friend,
For the lesson thou hast taught. --Longfellow.
His thanks, Her thanks, etc., of his or her own accord;
with his or her good will; voluntary. [Obs.]
Full sooth is said that love ne lordship,
Will not, his thanks, have no fellowship. --Chaucer.
In thank, with thanks or thankfulness. [Obs.]
Thank offering, an offering made as an expression of