"bottom of the human 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.
De Vaan has it from a PIE *se/ol-o- "place, habitation, human settlement," with cognates in Lithuanian sala "island, field surrounded by meadows, village;" Old Church Slavonic selo "field, courtyard, village," obsolete Polish siolo, Russian selo "village;" Old High Germansal "habitation, room;" Old Norse salr "hall, room, house."
In English, the meaning "bottom of a shoe or boot" is from late 14c.