Ungrateful
Part of speech: adjective
Definition: not grateful; not expressing gratitude; a dissatisfied person.
Example sentence: It's a cliche, but the people who enjoy your work and who come up and say, 'I enjoyed that and I liked that,' they are the people who ultimately are keeping you in work. And so, it would be rude and ungrateful of me to be anything but polite.
Unthankful
Part of speech: adjective
Definition: Not thankful; ungrateful.