‘Let's go’; (also) ‘come now’, ‘there you are’ (used by way of encouragement, or as a general expression of acceptance, resignation, etc.).
Mid 17th century; earliest use found in George Tooke (1595–1675), army officer and writer. From French allons, 1st person plural imperative of aller to go from post-classical Latin alare, ultimately a variant (with loss of the unstressed medial -u- and subsequent assimilation of consonants, although the exact phonological history is unclear) of classical Latin ambulāre to walk. See further Französisches etymol. Wörterbuch XXIV. at ambulare.