From The Martial Arts Encyclopedia

Bully is a tongue-in-cheek nickname for Bullshido.net forum members.

The context of the term is that it is used in a quasi-sincere attempt at semantic amelioration (referencing the early 19th century use of the word as a positive exclamation, most notably by Teddy Roosevelt); along with an implied, self-deprecating jab at the notion that Bullshido forums members are aggressive, mean, etc.

See also: Bullshidoka