1610s, "slope of the edge of a cutting tool," also "groove by which a stone is held in its setting," from Old French *besel (13c.; Modern French biseau), cognate with Spanish and Portuguese bisel; of uncertain origin, perhaps literally "a stone with two angles," from Vulgar Latin *bis-alus, from bis- "twice" (from PIE root *dwo- "two") + ala "wing, side" (see alar). Meaning "oblique face of a gem" is from c. 1840. The verb meaning "grind (a tool) down to an edge" is from 1670s. Compare bevel.