Is it ensure, insure, or assure?
1550s, "pleasing to the mind," also "full of gratitude," from obsolete adj. grate "agreeable, thankful," from Latin gratus "pleasing" (see grace). "A most unusual formation" [Weekley]. Hard to think of another case where English uses -ful to make an adjective from an adjective. Related: Gratefully; gratefulness.