1. Not feeling or showing gratitude
   "How sharper than a serpent's tooth it is / To have a thankless child!"
   - ungrateful, unthankful 
2. Not likely to be rewarded
   "grading papers is a thankless task"
   - unappreciated, ungratifying

See also: unappreciative, unrewarding

• sea mat
• sea mew
• sea mile
• sea milkwort
• sea moss
• sea mouse
• sea nymph
• Sea of Azof
• Sea of Azoff
• Sea of Azov
• Sea of Cortes
• Sea of Japan
• Sea of Marmara
• Sea of Marmora
• Sea of Okhotsk
• sea onion
• sea otter
• sea pea
• sea pen
• sea pink
• sea poacher
• sea poker
• sea poppy
• sea power
• sea raven