'legible' originates from Latin, specifically the word 'legibilis', where 'legere' meant 'to read' or 'to gather'.

'legible' came into English via Medieval Latin/Old French: Latin 'legibilis' -> Old French 'legible' -> Middle English 'legible', later forming the adverb 'legibly' with the suffix '-ly'.

Initially it meant 'able to be read' in the literal sense; over time this core meaning has remained largely the same (with extended uses meaning 'clear' or 'understandable'), and the adverb 'legibly' simply conveys that manner.