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

• odalisk
• odalisque
• odd
• odd fellow
• odd fish
• Odd Hassel
• odd lot
• odd man out
• odd one out
• odd-even check
• odd-job
• odd-job man
• odd-leg caliper
• odd-pinnate
• odd-pinnate leaf
• odd-toed ungulate
• oddball
• oddish
• oddity
• oddly
• oddment
• oddments
• oddness
• Oddone Colonna
• odds