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 also had occasionary "affording opportunity, favorable" (mid-15c.).
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.
Others are reading
Definitions of occasionally from WordNet
sporadically and infrequently;
he was arrogant and occasionally callous
open areas are only occasionally interrupted by clumps of trees
Synonyms: on occasion / once in a while / now and then / now and again / at times / from time to time