gratefulyoudaoicibaDictYouDict[grateful 词源字典]
grateful: [16] Grateful is a curious sort of adjective. The grate that a grateful person is full of is a now obsolete adjective, meaning ‘pleasing’ and ‘thankful’, which was derived from Latin grātus. It is unusual for adjectives ending in -ful themselves to be formed from adjectives, rather than from nouns, and it has been suggested in this case that the related Italian gradevole ‘pleasing’ may have had some influence.

Latin grātus itself meant ‘pleasing’ as well as ‘thankful’, and has also given English congratulate [16], gratify [16], gratitude [16], and gratuity [16], and, via the derived noun grātia, grace and gratis [15].
=> congratulate, grace, gratis, gratitude[grateful etymology, grateful origin, 英语词源]
grateful (adj.)youdaoicibaDictYouDict
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]