"great beyond measure," early 15c., from Old French immense (mid-14c.), from Latin immensus "immeasurable, boundless," also used figuratively, from assimilated form of in- "not, opposite of" (see in- (1)) + mensus "measured," past participle of metiri "to measure" (from PIE root *me- (2) "to measure"). A vogue word in 18c., and mocked as such:
For instance, a long while every thing was immense great and immense little, immense handsome and immense ugly. Miss Tippet from the cloisters, could not drink tea with Master Parchment at the White Conduit-house, unless it was an immense fine day, yet probably it might rain so immense, there was no going without a coach. ["Town and Country Magazine" (in "Annual Register" for 1772)]
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.
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Definitions of immensely from WordNet
to an exceedingly great extent or degree;
was immensely more important to the project as a scientist than as an administrator