adverb (r)

• unmistakably (`plain' is often used informally for `plainly')(adv.all)

  Example:
  You are plainly wrong.
  

  Synonym:
  apparently, evidently, manifestly, obviously, patently, plain
  

  source: wordnet30
• in a simple manner; without extravagance or embellishment(adv.all)

  Example:
  She was dressed plainly.
  

  source: wordnet30
• In a plain manner; clearly.(adverb)

  source: webster1913