Aug 21, 2025 irrational i-ˈra-sh(ə-)nəl 4 syllables uncommon ‹ adjective. not rational: such as irrational fearsan irrational root of an equation adjective. lacking usual or normal mental clarity or coherence a length that is irrational adjective. not endowed with reason or understanding noun. an irrational being noun. irrational number › Origin Middle English, from Latin irrationalis, from in- + rationalis rational.