legend (n.) Look up legend at Dictionary.com
early 14c., "narrative dealing with a happening or an event," from Old French legende (12c., Modern French légende) and directly from Medieval Latin legenda "legend, story," literally "(things) to be read," on certain days in church, etc., from Latin legendus, neuter plural gerundive of legere "to read, gather, select" (see lecture (n.)).

Used originally of saints' lives; extended sense of "nonhistorical or mythical story" first recorded late 14c. Meaning "writing or inscription" (especially on a coin or medal) is from 1610s; on a map, illustration, etc., from 1903.