irrationality(n = noun.state) unreason - the state of being irrational; lacking powers of understanding;
is a kind of insanity
Derived form adjective irrational₁

   The quality or state of being irrational; absence of rationality. South. [1913 Webster]

N  imbecility folly, want of intelligence &c, want of intellect, shadowness, silliness, foolishness, imbecility, incapacity, vacancy of mind, poverty of intellect, weakness of intellect, clouded perception, poor head, apartments to let, stupidity, stolidity, hebetude, dull understanding, meanest capacity, shortsightedness, incompetence, one's weak side, not one's strong point, bias, infatuation, simplicity, puerility, babyhood, dotage, anility, second childishness, fatuity, idiocy, idiotism, driveling, folly, frivolity, irrationality, trifling, ineptitude, nugacity, inconsistency, lip wisdom, conceit, sophistry, giddiness, eccentricity, extravagance, rashness, act of folly, unintelligent, unintellectual, unreasoning, mindless, witless, reasoningless, brainless, halfbaked, having no head, not bright, inapprehensible, weak headed, addle headed, puzzle headed, blunder headed, muddle headed, muddy headed, pig headed, beetle headed, buffle headed, chuckle headed, mutton headed, maggoty headed, grossheaded, beef headed, fat witted, fat-headed, weak-minded, feeble-minded, dull minded, shallow minded, lack- brained, rattle-brained, rattle headed, half witted, lean witted, short witted, dull witted, blunt-witted, shallow-pated, clod-pated, addle- pated, addle-brained, dim-sighted, short-sighted, thick-skulled, weak in the upper story, shallow, borne, weak, wanting, soft, sappy, spoony, dull, dull as a beetle, stupid, heavy, insulse, obtuse, blunt, stolid, doltish, asinine, inapt, prosaic, hebetudinous, childish, child-like, infantine, infantile, babyish, babish, puerile, anile, simple, old-womanish, fatuous, idiotic, imbecile, driveling, blatant, babbling, vacant, sottish, bewildered, blockish, unteachable Boeotian, Boeotic, bovine, ungifted, undiscerning, unenlightened, unwise, unphilosophical, apish, simious, foolish, silly, senseless, irrational, insensate, nonsensical, inept, maudlin, narrow-minded, bigoted, giddy, rash, eccentric, foolish, unwise, injudicious, improper, unreasonable, without reason, ridiculous, absurd, idiotic, silly, stupid, asinine, ill-imagined, ill-advised, ill-judged, ill-devised, mal entendu, inconsistent, irrational, unphilosophical, extravagant, sleeveless, idle, pointless, useless, inexpedient, frivolous, Davus sum non, a fool's bolt is soon shot clitellae bovi sunt im, fools rush in where angels fear to tread, il n' a ni bouche ni eperon, the bookful blockhead, ignorantly read, to varnish nonsense with the charms of sound.