/ ɪˈræʃənl; NAmE / adjective
not based on, or not using, clear logical thought
SYN unreasonable :
an irrational fear
You're being irrational.
OPP rational
► ir·ration·al·ity / ɪˌræʃəˈnæləti; NAmE / noun [ U , C , usually sing. ]
► ir·ration·al·ly / ɪˈræʃnəli; NAmE / adverb :
to behave irrationally
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WORD ORIGIN
late Middle English : from Latin irrationalis , from in- not + rationalis (from ratio(n-) reckoning, reason, from rat- reckoned, from the verb reri ).