1833, American English, in countrified humor writing of "Major Jack Downing" of Maine (Seba Smith), "stylish, splendid, fine;" probably a colloquial alteration (intensification) of sumptuous. By late 19c. especially of food, "delicious, delightful," and it was noted 1890s and early 20c. as a vogue word among college girls (also as scrum, scrummy). Related: Scrumptiously; scrumptiousness.
OED (2nd edition, print) has scrumptious as probably identical with dialectal scrumptious "mean, stingy, close-fisted," and ultimately related to shrimp. The editors insist the sense transition "is not impossible," and they compare nice.