As Flies to wanton Boyes are we to th' Gods, They kill vs for their sport. [Shakespeare, "Lear," 1605]
Especially of sexual indulgence from late 14c. The only English survival of a once-common Germanic negating prefix still active in Dutch (cf. wanbestuur "misgovernment," wanluid "discordant sound"), German (wahn-), etc. Related: Wantonly; wantonness.