"thankless" in the adjective sense
1. ungrateful, thankless, unthankful
not feeling or showing gratitude
"ungrateful heirs"
"How sharper than a serpent's tooth it is / To have a thankless child!"- Shakespeare
2. thankless, unappreciated, ungratifying
not likely to be rewarded
"grading papers is a thankless task"
Source: WordNet® (An amazing lexical database of English)
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