Fleetingly/ˈfliːtɪŋli/ Definition 1 of 1Pro Tip 1/3Emotional Impact Indicates a brief emotional exchange or realization that leaves a lasting impression. He smiled at her fleetingly, yet it warmed her heart for days. Pro Tip 2/3Quick Transition Used to describe events that happen rapidly, transitioning quickly from one state to another. The landscape changed fleetingly as the train sped by. Pro Tip 3/3Not Superficial Although brief, it does not imply something is unimportant or superficial. Her fleetingly expressed concerns hinted at deeper issues.