徒然 is literary/written register. In casual speech, Chinese speakers say 白白 (báibái) or 白费 (báifèi) instead — e.g. 白等了一天 sounds more natural in conversation than 徒然等了一天.
memory
Here 徒 means 'merely / only' (as in 徒劳 'in vain labor'), and 然 is the classical adverb suffix '-ly'. So 徒然 ≈ 'merely-ly' → for nothing.
