ungrateful adj.

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

1550s, "pleasing to the mind," also "full of gratitude, disposed to repay favors bestowed," from obsolete adjective grate "agreeable, pleasant," from Latin gratus "pleasing" (from suffixed form of PIE root *gwere- (2) "to favor"). "A most unusual formation" [Weekley]. A rare, irregular case of English using -ful to make an adjective from an adjective (the only other one I can find is direful "characterized by or fraught with something dreadful," 1580s). 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]