Old English þanc, þonc in late use "grateful thought, gratitude," plural form thanks from mid-13c., from the same root as thank (v.). Compare Old Saxon thank, Old Frisian thank, Old Norse þökk, Dutch dank, German Dank. The Old English noun originally and chiefly meant "thought, reflection, sentiment; mind, will, purpose;" also "grace, mercy, pardon; pleasure, satisfaction."
As short for I give you thanks from 1580s; often with extensions, such as thanks a lot (1908). Spelling thanx attested by 1907.