1735, "unable or unwilling to endure" (a condition, etc.), from Latin intolerantem (nominative intolerans) "not enduring, impatient, intolerant; intolerable," from in- "not" (see in- (1)) + tolerans, present participle of tolerare "to bear, endure" (see toleration).
Meaning "not disposed to endure contrary opinions or beliefs, impatient of dissent or opposition" is from 1765. Of plants, with reference to deep shade, from 1898. The noun meaning "person or persons who do not favor toleration" is from 1765. Related: Intolerantly.