"unlooked-for acquisition or good fortune," 1812, earlier "a shipwreck" (from the perspective of people living along the coast), by 1806, from Middle English Godes sonde (c. 1200) "God's messenger; what God sends, gift from God, happening caused by God," from God + send (n.) "a message, a message," literally "that which is sent," from Middle English sonde, from Old English sand, the noun associated with sendan (see send (v.)). The spelling was conformed to the verb in later Middle English.
The common people in Cornwall call, as impiously as inhumanely, a shipwreck on their shores, "a Godsend." [Rev. William Lisle Bowles, footnote in "The Works of Alexander Pope," London, 1806]