You’re a towel!
“A towel is just about the most massively useful thing any interstellar Hitchhiker can carry. Partly it has great practical value. You can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a miniraft down the slow heavy River Moth; wet it for use in hand-to-hand combat; wrap it round your head to ward off noxious fumes or avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mind-bogglingly stupid animal, it assumes that if you can’t see it, it can’t see you — daft as a brush, but very very ravenous); you can wave your towel in emergencies as a distress signal, and of course you can dry yourself off with it if it still seems to be clean enough.”
— The Hitchhiker’s Guide to the Galaxy
Hence a phrase that has passed into hitchhiking slang, as in “Hey, you sass that hoopy Ford Prefect? There’s a frood who really knows where his towel is.”