"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). Adv. solely is attested from 1495.
"flatfish," 1252, 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.