Why is the ninth month called September?
1610s, "sloping edge," also "groove in which a stone is set," from Old French *besel (13c.; Modern French biseau), cognate with Spanish bisel; of uncertain origin, perhaps literally "a stone with two angles," from Vulgar Latin *bis-alus, from bis- "twice" (see bis-) + 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.