alacrity n.

"liveliness, briskness," mid-15c., from Latin alacritatem (nominative alacritas) "liveliness, ardor, eagerness," from alacer (genitive alacris) "cheerful, brisk, lively;" a word of uncertain origin. Perhaps cognate with Gothic aljan "zeal," Old English ellen "courage, zeal, strength," Old High German ellian. But de Vaan suggests the root sense is "to wander, roam" and sees a possible connection with ambulare.