RFL is an abbreviation you may have noticed when looking at Darn Tough socks. RFL stands for Real-F'in-Light, as these socks are constructed from thin, ultralight Merino wool and are ideal for wearing in ski boots. , The Darn Tough Vermont RFL sock is the lightest ski sock in the brand's lineup. RFL is not a specific technical term among sock companies, just a DTV sock name that stands for "really freakin' light."