irrational

adjective
ir·​ra·​tio·​nal | \i-ˈra-sh(ə-)nəl, ˌi(r)-\

Definition of irrational 

(Entry 1 of 2)

: not rational: such as

a(1) : not endowed with reason or understanding

(2) : lacking usual or normal mental clarity or coherence

b : not governed by or according to reason irrational fears

c Greek & Latin prosody

(1) of a syllable : having a quantity other than that required by the meter

(2) of a foot : containing such a syllable

d(1) : being an irrational number an irrational root of an equation

(2) : having a numerical value that is an irrational number a length that is irrational

irrational

noun

Definition of irrational (Entry 2 of 2)

1 : an irrational being

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Other Words from irrational

Adjective

irrationality \ i-​ˌra-​shə-​ˈna-​lə-​tē , ˌi(r)-​ \ noun
irrationally \ i-​ˈra-​sh(ə-​)nə-​lē , ˌi(r)-​ \ adverb

Examples of irrational in a Sentence

Adjective

He became irrational as the fever got worse. She had an irrational fear of cats.

Recent Examples on the Web: Adjective

Besides, throwing out the lamp would have been even more irrational than my initial purchase. Alexis Cheung, Curbed, "Love, loss, and my Noguchi lamp," 22 June 2018 In fact, some of his followers are actively hostile to religion, seeing it as fundamentally irrational. Zack Beauchamp, Vox, "Jordan Peterson, the obscure Canadian psychologist turned right-wing celebrity, explained," 21 May 2018 That’s not so far off, according to Duke psychologist and behavioral economist Dan Ariely, famous for his research into the irrational ways that people behave. Janice Kaplan With Barnaby Marsh, The Cut, "What People Who Are ‘Lucky in Love’ All Have in Common," 21 Mar. 2018 The similarity, rather, lies in the way an apparently irrational decision can be made logical and necessary by a certain kind of twisted internal logic that can escape outsiders. Jonathan Chait, Daily Intelligencer, "Trump Is Taking Out His Enemies And Turning Toward Robert Mueller," 17 Mar. 2018 Just stuff that is maybe a little irrational and not important at that time. Jeff Sanders, sandiegouniontribune.com, "Talking with ... Padres pitcher Robbie Erlin," 22 June 2018 The disparity in assumptions about this series is irrational and out of alignment. Greg Cote, miamiherald, "The Miami Heat is about to shock the 'it'-team 76ers in the playoffs, and here's why | Miami Herald," 12 Apr. 2018 The duo pioneered the field of behavioral economics, which attempts to explain how psychological and emotional factors lead to irrational decision making and market inefficiencies. Kathleen Pender, San Francisco Chronicle, "Author Michael Lewis turns his attention from Wall Street to Washington, D.C.," 10 Apr. 2018 This raised the question of whether the process is irrational or is, in some way, a purposeful evolutionary advantage. Maya Miller, Scientific American, "Why It’s So Hard to Junk Bad Decisions—Edging Closer to Understanding “Sunk Cost”," 12 July 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'irrational.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of irrational

Adjective

14th century, in the meaning defined above

Noun

1646, in the meaning defined at sense 1

History and Etymology for irrational

Adjective

Middle English, from Latin irrationalis, from in- + rationalis rational

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Last Updated

5 Nov 2018

Look-up Popularity

Time Traveler for irrational

The first known use of irrational was in the 14th century

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More Definitions for irrational

irrational

adjective

English Language Learners Definition of irrational

: not thinking clearly : not able to use reason or good judgment

: not based on reason, good judgment, or clear thinking

irrational

adjective
ir·​ra·​tio·​nal | \i-ˈra-shə-nᵊl \

Kids Definition of irrational

1 : not able to reason The fever made him irrational.

2 : not based on reason irrational fears

Other Words from irrational

irrationally adverb

irrational

adjective
ir·​ra·​tio·​nal | \(ˈ)ir-ˈ(r)ash-nəl, -ən-ᵊl \

Medical Definition of irrational 

: not rational: as

a : lacking usual or normal mental clarity or coherence was irrational for several days after the accident

b : not governed by or according to reason irrational fears

Other Words from irrational

irrationality \ (ˌ)ir-​ˌ(r)ash-​ə-​ˈnal-​ət-​ē \ noun plural -ties
irrationally \ (ˈ)ir-​ˈ(r)ash-​nə-​lē, -​ən-​ᵊl-​ē \ adverb

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irrational

adjective
ir·​ra·​tio·​nal | \ir-ˈra-shə-nəl \

Legal Definition of irrational 

: not rational: as

a : not governed by reason, mental clarity, or understanding

b : not governed by a fair consideration of facts or evidence broadly : arbitrary an irrational decision to deny the permit

Other Words from irrational

irrationality \ ir-​ˌra-​shə-​ˈna-​lə-​tē \ noun
irrationally adverb

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