"bottom of the foot" ("technically, the planta, corresponding to the palm of the hand," Century Dictionary), early 14c., from Old French sole, from Vulgar Latin *sola, from Latin solea "sandal, bottom of a shoe; a flatfish," from solum "bottom, ground, foundation, lowest point of a thing" (hence "sole of the foot"), a word of uncertain origin. In English, the meaning "bottom of a shoe or boot" is from late 14c.
"single, alone, having no husband or wife; one and only, singular, unique," late 14c., from Old French soul "only, alone, just," from Latin solus "alone, only, single, sole; forsaken; extraordinary," of unknown origin, perhaps related to se "oneself," from PIE reflexive root *swo- (see so).
common European flatfish, mid-13c., from Old French sole, from Latin solea "a kind of flatfish," originally "sandal" (see sole (n.1)); so called from resemblance of the fish to a flat shoe.
muscle of the calf of the leg, 1670s, Modern Latin, from Latin solea "sole" (see sole (n.1)). So called for its flatness.
mid-14c., "alone, living alone," from Old French solitaire, from Latin solitarius "alone, lonely, isolated," from solitas "loneliness, solitude," from solus "alone" (see sole (adj.)). Meaning "single, sole, only" is from 1742. Related: Solitarily; solitariness. As a noun from late 14c.; from 1854 as short for solitary confinement (that phrase recorded from 1690s).
Latin, "alone" (see sole (adj.)), used in stage directions by 1590s. Masculine; the fem. is sola, but in stage directions solus typically serves for both. Also in phrases solus cum sola "alone with an unchaperoned woman" and solus cum solo "all on one's own," both literally meaning "alone with alone."