frustra

Frustra is an adverb and stays an adverb: it has no case, no gender and no comparative, and it normally stands first in its clause. The related noun is fraus, fraudis, "deceit", which tells you the original sense: not merely fruitless but deceived, cheated of the result you expected.

Two things to note. Frustra describes the effort, not the thing, so you cannot use it where English says "a vain hope"; that is spes vana. And it is one of the few Latin words that survives almost unchanged into scholarly English, where a frustrated attempt still carries the Latin sense of one that came to nothing rather than one that annoyed somebody. Frustror, frustrari, "I cheat, disappoint", is the deponent verb behind frustrate.