Is it ensure, insure, or assure?
1833, in countrified humor writing of "Major Jack Downing" of Maine (Seba Smith); probably a colloquial alteration of sumptuous. Originally "stylish, splendid;" sense of "delicious" is by 1881. Related: Scrumptiously; scrumptiousness.
Excellent; superior; luxurious: What a scrumptious dessert!
[1836+; perhaps a humorous alteration of sumptuous]