Roget's Thesaurus: Entry 471 (Impossibility)

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#471. Impossibility. -- N. impossibility &c. adj.; what cannot, what can never be; sour grapes; hopelessness &c. 859.

V. be impossible &c. adj.; have no chance whatever.

attempt impossibilities; square the circle, wash a blackamoor white; 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[Fr], extract sunbeams from cucumbers, set the Thames on fire, milk a he-goat into a sieve, catch a weasel asleep, rompre l'anguille au genou[Fr], be in two places at once.

Adj. impossible; not possible &c. 470; absurd, contrary to reason; unlikely; unreasonable &c. 477; incredible &c. 485; beyond the bounds of reason, beyond the bounds of possibility, beyond the realm of possibility; from which reason recoils; visionary; inconceivable &c. (improbable) 473; prodigious &c. (wonderful) 870; unimaginable, inimaginable[obs]; unthinkable.

impracticable unachievable; unfeasible, infeasible; insuperable; unsurmountable[obs], insurmountable; unattainable, unobtainable; out of reach, out of the question; not to be had, not to be thought of; beyond control; desperate &c. (hopeless) 859; incompatible &c. 24; inaccessible, uncomeatable[obs], impassable, impervious, innavigable[obs], inextricable; self-contradictory.

out of one's power, beyond one's power, beyond one's depth, beyond one's reach, beyond one's grasp; too much for; ultra crepidam[Lat].

Phr. the grapes are sour; non possumus[Lat]; non nostrum tantas componere lites [Lat][Vergil]; look for a needle in a haystack, chercher une aiguille dans une botte de foin [obs][Fr.]; il a le mer  boire[obs].