adverb (r)

• continuing forever without end(adv.all)

  Synonym:
  infinitely
  Antonym:
  finitely
  

  source: wordnet30
• with unflagging resolve(adv.all)

  Synonym:
  ceaselessly, continuously, incessantly, unceasingly, unendingly
  

  source: wordnet30
• (spatial sense) without bounds(adv.all)

  Example:
  The Nubian desert seemed to stretch out before them endlessly.
  

  source: wordnet30
• all the time; seemingly without stopping(adv.all)

  Example:
  Her nagging went on endlessly.
  

  Synonym:
  interminably
  

  source: wordnet30
• In an endless manner.(adverb)

  source: webster1913