Insulting a Viking is comparable to taunting a teenage boy, but with greater consequences.
The greatest insult was to suggest that a man was sansorðinn — “used in the position of a female (blauðr) by another man.”
The consequence of this could be fullrettirsorð “full penalty,” in which the insulted man could kill the insulter without recourse. The authorities would turn a blind eye to this.
Indirect insults, which nonetheless have the same connotations, include calling a man a “mare,” or a “woman,” or argr (generally meaning “cowardly” but in a sexual context meaning “emasculated, unmanned, womanish”). This could also result in fullrettirsorð.
Argr is frequently used in norse literature.