grateful (adj.) Look up grateful at
1550s, "pleasing to the mind," also "full of gratitude, disposed to repay favors bestowed," from obsolete adjective grate "agreeable, pleasant," from Latin gratus "pleasing" (see grace (n.)). "A most unusual formation" [Weekley]. Is there another case where English uses -ful to make an adjective from an adjective? Related: Gratefully (1540s); gratefulness.
Grateful often expresses the feeling and the readiness to manifest the feeling by acts, even a long time after the rendering of the favor; thankful refers rather to the immediate acknowledgment of the favor by words. [Century Dictionary]