unrecognized, unappreciated, or unrewarded, not likely to give one pleasure or invite the gratitude of others, typically describing a task that is unpleasant and/or difficult;
e.g. He put in hours of thankless effort, only for all of his work to be insulted and destroyed.
(adjective)
ungrateful or unappreciative, not tending to feel thanks or express gratitude, describing a person or their attitude;
e.g. She seemed to be thankless and indifferent despite all of their help.