"pleasant to the taste," 1560s, from -some (1) + tooth in a figurative sense of "appetite, taste, liking" attested from late 14c. (compare sweet tooth, also figurative use of palate). The extended sense of "attractive" (1550s) is attested earlier. Related: Toothsomely; toothsomeness.
[toothsome etymology, toothsome origin, 英语词源]