1721 as a musical term, from Italian allegro "brisk, sprightly, cheerful," from Latin alacrem (nominative alacer) "lively, cheerful, brisk" (see alacrity). The same Latin word came into English 17c. as aleger "lively, brisk," from Old French alegre, from Latin Related: alacris; and Milton used "L'Allegro" in its literal sense as a poem title (1632).
