A person who uses strength or influence to harm or intimidate others who are weaker. Recorded from the mid 16th century (probably coming from Middle Dutch boele ‘lover’), the original use was as a term of endearment applied to either sex; later becoming a familiar form of address to a male friend. The informal North American use of bully as an adjective to mean ‘first rate’ derives ultimately from this.
a bully is always a coward traditional association of overbearing behaviour with lack of fortitude when challenged; recorded from the early 19th century.
See also bully pulpit.