The Collaborative International Dictionary of English v.0.48:

Manslayer \Man"slay`er\, n.
   One who kills a human being; one who commits manslaughter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

manslayer
    n 1: a criminal who commits homicide (who performs the unlawful
         premeditated killing of another human being) [syn:
         murderer, liquidator, manslayer]

Easton's 1897 Bible Dictionary:

Manslayer
   one who was guilty of accidental homicide, and was entitled to
   flee to a city of refuge (Num. 35:6, 12, 22, 23), his compulsory
   residence in which terminated with the death of the high priest.
   (See CITY OF REFUGE.)