futility

be impossible; have no chance whatever., attempt impossibilities; square the circle, get blood from a stone, get blood from a turnip; look for a needle in a haystack; skin a flint; make a silk purse out of a sow's ear, make bricks without straw; have nothing to go upon; weave a rope of sand, build castles in the air, prendre la lune avec les dents, extract sunbeams from cucumbers, set the Thames on fire, fetch water in a sieve, milk a he-goat into a sieve, catch a weasel asleep, rompre l'anguille au genou, be in two places at once.