Etymology
Advertisement
irrational (adj.)
late 15c., "not endowed with reason" (of beasts, etc.), from Latin irrationalis/inrationalis "without reason, not rational," from assimilated form of in- "not, opposite of" (see in- (1)) + rationalis "of or belonging to reason, reasonable" (see rational (adj.)).

Meaning "illogical, absurd" is attested from 1640s. Related: Irrationally. The mathematical sense "inexpressible in ordinary numbers" is from late 14c. in English, from use of the Latin word as a translation of Greek alogon in Euclid.
Related entries & more 
Advertisement
irrationality (n.)
1560s, originally in the mathematical sense, from irrational + -ity. Meaning "unreasonableness, absurdity" is from 1640s.
Related entries & more 
-phobia 

word-forming element meaning "excessive or irrational fear, horror, or aversion," from Latin -phobia and directly from Greek -phobia "panic fear of," from phobos "fear" (see phobia). In widespread popular use with native words from c. 1800. In psychology, "an abnormal or irrational fear." Related: -phobic.

Related entries & more 
monophobia (n.)

"morbid dread of being left alone," 1879, from mono- "alone" + -phobia "irrational fear of." Related: Monophobic.

Related entries & more 
madness (n.)

late 14c., "insanity, lunacy, dementia; rash or irrational conduct, headstrong passion, extreme folly," from mad (adj.) + -ness. Sense of "foolishness" is from early 15c.

Related entries & more 
Advertisement
unreasonable (adj.)
mid-14c., "irrational, illogical," from un- (1) "not" + reasonable. From late 14c. as "excessive, going beyond what is sensible or realistic." Related: Unreasonably; unreasonableness.
Related entries & more 
edgy (adj.)
"having sharp edges," 1755, from edge (n.) + -y (2). Meaning "tense and irritable" is attested by 1837, perhaps from notion of being on the edge, at the point of doing something irrational (a figurative use attested from c. 1600). Related: Edgily; edginess.
Related entries & more 
mindless (adj.)

c. 1400, "unmindful, heedless, negligent," also "senseless, beside oneself, irrational, wanting power of thought," from mind (n.) + -less. Related: Mindlessly; mindlessness. Old English had myndleas "foolish, senseless."

Related entries & more 
insensate (adj.)

1510s, "lacking or deprived of physical senses," from Late Latin insensatus "irrational, foolish," from in- "not" (see in- (1)) + sensatus "gifted with sense" (see sensate).

Meaning "irrational, maniacal, lacking or deprived of mental sense" is from 1520s; meaning "lacking or deprived of moral sense, unfeeling" is from 1550s. Insensate means "not capable of feeling sensation," often "inanimate;" insensible means "lacking the power to feel with the senses," hence, often, "unconscious;" insensitive means "having little or no reaction to what is perceived by one's senses," often "tactless." Related: Insensately; insensateness.

Related entries & more 
enthusiastic (adj.)
c. 1600, "pertaining to possession by a deity," from Greek enthousiastikos "inspired," from enthousiazein "be possessed or inspired by a god" (see enthusiasm). Meaning "pertaining to irrational delusion in religion" is from 1690s. The main modern sense, in reference to feelings or persons, "intensely eager, rapturous," is from 1786. Related: Enthusiastically.
Related entries & more