'sole' (modern noun/adjective) ultimately originates from Latin: for the shoe/fish sense from Latin 'solea' (meaning 'sandal' or 'flat fish'), and for the 'only' sense from Latin 'solus' (meaning 'alone').

'sole' (shoe/fish) passed into English via Old French 'sole' from Latin 'solea'; the word entered Middle English as 'sole' and developed the meanings 'sandal sole', 'flat fish', and the bottom of the foot. The adjective 'sole' came from Latin 'solus' through Old French 'seul' into Middle English.

Initially, 'solea' referred to a sandal (and the flat fish named for its shape); over time the word broadened to mean the bottom of the foot and the bottom of a shoe. The adjective derived from 'solus' originally meant 'alone' and evolved into the modern sense 'only' or 'single'.