means A traditional story handed down as history but not verifiable, or by extension a person or deed so famous it has passed into such storytelling — also the explanatory key on a map or coin.

from From Latin 'legenda,' meaning 'things to be read,' from 'legere,' 'to read' (also a cousin of 'lecture' and 'legible'). In the Middle Ages a 'legend' was specifically a saint's life read aloud on their feast day — text meant for recitation. Over time the holy reading drifted toward the embellished and the doubtful, until 'legend' came to mean a tale too good to fully trust. The map-key sense kept the older, literal thread: the words you're meant to read to make sense of the picture.