"bottom of the foot," early 14c., from O.Fr. sole, from L. solea "sandal, bottom of a shoe," from solum "bottom, ground, soil," of unknown origin. The verb meaning "to provide with a sole" is recorded from 1560s.
"single," late 14c., from O.Fr. soul (fem. soule), from L. solus "alone," of unknown origin, perhaps related to se "oneself," from PIE reflexive base *swo- (see so).
"flatfish," mid-13c., from O.Fr. sole, from L. solea "a kind of flatfish," originally "sandal" (see sole (n.1)), so called from resemblance of the fish to a sandal.