Adjective thankless has 2 senses
  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"
    Antonym: rewarding (indirect, via unrewarding)
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