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

• dark-field microscope
• dark-fruited
• dark-gray
• dark-green
• dark-grey
• dark-haired
• dark-skinned
• dark-spotted
• darken
• darkened
• darkener
• darkening
• darkey
• darkie
• darkish
• darkle
• darkling
• darkling beetle
• darkling groung beetle
• darkly
• darkness
• darkroom
• darksome
• darky
• darling