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Webster 1913 Edition

Noun.

One who backbites; a secret calumniator or detractor.

Noun.

One who slanders, calumniates or speaks ill of the absent.

See also: back biter

backbiter ‎(plural backbiters)

person who says nasty things about another person behind the second person's back: that is, out of sight and hearing of the second person.