"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)]