late 14c., "occurring now and then," from occasion (n.) + -al (1) or from Old French ocasionel and directly from Medieval Latin occasionalis. Meaning "casual" is 1560s. Meaning "happening on or pertaining to a particular occasion" is from 1630s. Of furniture, etc., "adapted for use on special occasions," from 1749. Middle English had also occasionary "affording opportunity, favorable" (mid-15c.).