Insulting a Viking is easy: Vikings, like 14-year-old boys, almost always used sexual put-downs. But beware of the consequences.
The worst was implying that a man was sansorðinn -- "used in the position of a female (blauðr) by another man," in other words "demonstrably sodomized." That could be punished by fullrettirsorð "full penalty," meaning that the insulted man could kill the insulter with impunity. The authorities would look the other way. The insult didn't have to be that direct, as long as the meaning was clear. Calling a man a "mare," or a "woman," or worse, argr (its polite meaning is "cowardly"; its sexual meaning is "emasculated, unmanned, womanish") could also call down the weight of fullrettirsorð. In the Lokasenna ("The Insolence of Loki"), the term argr is bandied about openly.
In modern American conceptions of masculinity, any suggestion of "homosexuality" is considered equally insulting. But note that the Vikings distinguished the "catcher" from the "pitcher" role. This they had in common with a great many Western and other cultures throughout history. Male sexuality, the urge to copulate, was presumed to be powerful and general and not particularly or necessarily limited to women. The strict division of male sexuality into hetero- and homo- that we presume today was not usually made before the late 19th century. Attempts to fit Walt Whitman or Lord Byron into a comfortable diagnosis of "homosexual" falter on the realities of their time and place.
Nonetheless, for Vikings, a muttered accusation of sansorðinn was serious business. The intensity of their rages and passion is sometimes easy to overlook for those who study their literature, for people in the sagas almost never talk in depth about what they are feeling and words for emotional states are rare. Characters' behavior often appears bizarrely unemotional. Talking with the man who killed her beloved husband, a woman makes jokes about the bloody ax he carries. When a warrior hears that an enemy has died, instead of celebrating, he takes to his bed for several months. But to lap the distance between that world and ours, you have to keep reading, and think in terms of the codes of honor. The widow's laughter while talking with her husband's killer is a nervous outburst, perhaps, or even a ploy; soon she tricks him with "advice" that will lead to his own death. A Viking becomes depressed when an old enemy passes away, for that means he will never be able to repay the insults he has suffered.
We are accustomed to understanding emotion as a personal experience -- something that occurs "inside" and may or may not be expressed. But there are cultures in which emotion is overwhelmingly a social matter, not a private one. The early use of humiliation referred not to an inner state but to being made humble in the presence of those higher on the social scale. Only in the 18th century did it become normal to say "I feel humiliated" rather than "I am humiliated."
There was so much emotion about Viking sexual insults that the thirteenth-century Gulaðing proscribed the use of these words in public. Among more educated Vikings sexual name-calling continued, often wrapped in sometimes obscure mythological or literary allusions. The stealth version includes trenið, or "wood-insult," which was an alternative of muðnið or verbal insult. If you wanted to imply that a man was sansorðin, you posted two figures made out of wood, one behind the other in a suggestive position, and you put the skull of a mare on the figure in front. (Think about it.)
In war, of course, all rules are lifted, especially when you are trying to terrorize your foes, so muðnið likely was rampant on the battlefield.