The Collaborative International Dictionary of English v.0.48:
Harasser \Har"ass*er\ (-[~e]r), n. One who harasses. [1913 Webster]WordNet (r) 3.0 (2006):
harasser n 1: a persistent tormentor 2: a persistent attacker; "the harassers were not members of the regular army" [syn: harasser, harrier]