1530s, "the giving of thanks," from thanks (n.) + giving. The noun thanks is attested from mid-14c., from the verb thank. In the specific sense of "public celebration acknowledging divine favors" thanksgiving dates from 1632 (the first one in America was held October 1621
by Plymouth Colony Pilgrims in appreciation of assistance from members of the Massasoit tribe and celebration of the first harvest); though Thanksgiving Day itself is not attested until 1674.