Quiz: Remember the definition of mal de mer?
"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"), of uncertain origin. In English, the meaning "bottom of a shoe or boot" is from late 14c.
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.
"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).
"furnish (a shoe) with a sole," 1560s, from sole (n.1). Related: Soled; soling.
The underside of the foot.