1530s, “the giving of thanks, action of expressing gratitude for favors, etc.,” from thanks (n.) + present participle of give (v.). The verbal phrase give thanks is attested from late 14c.; Wycliffe (c. 1400) has doinge of thankes for “thanksgiving.” A verb thanksgive (1630s) is a back-formation from the noun. The earlier noun was simply thanking “act or action of giving thanks” (late 13c.).
In the specific sense of “public celebration acknowledging divine favors” thanksgiving dates from 1630s (the traditional first such 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 1670s.
Source: O
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