When reading up on Norse society, you get the impression that a typical Norseman would feel right at home at a redneck bar. Far from the lagom and IKEA of their descendants, the Norse were a rough and tumble bunch of people, even after the introduction of Christianity into Scandinavia (see: Saint Olaf). Sagas and eddas abound with accounts of manslaughter, ass-slapping and general brawling; even their gods got in on the action.
The gravest insult to a Norseman was to call him a niðingr, a term which would find its closest equivalent to 'dirty rotten scoundrel'. This concept is commonly confused with another concept in Norse culture called ergi, which has connotations of lasciviousness and lack of self-control. Some sources (cited below) claim that both terms were used to describe mad Viking-on-Viking action. These claims are rather controversial, and there is no consensus among historians as to whether or not this was indeed the case. But, for the sake of cheeky exposition, I'll run with those claims (with the usual disclaimers that any sources on Norse sexuality are rather sketchy at best).
Norse sexuality based itself on an active-passive paradigm: there was always a pitcher and a catcher, and you only accused a Norseman of being a catcher at your own peril. Similar to Roman sexuality, Norse sexuality was more about power than purity. You didn't defile yourself by engaging in homosexual acts; you defiled yourself by being someone's bitch.
Niðr Here nor There
Central to this paradigm was the concept of nið, a directional derivation of which (niðr) is cognate to our word 'nether' (as in 'low'). Similar cognates exist in most other Germanic languages: Swedish ner, German nieder and Dutch neder, all meaning 'lower' or 'down'. Nið was, in brief, a state of low social status and esteem. A person in nið was called a niðing. To call a free Norseman a niðing was tantamount to fightin' words.
Nið was also used as a curse. In the Saga of Egill Skallagrímsson, after Egill was banished by King Eirik Bloodaxe, he erected a niðstang bedecked with runic curses and topped with the severed head of a horse pointed at Eirik's castle. Now that's metal. The word niðstang is translated as 'cursing-pole'.
A Friend in Nið is a Friend Indeed
This concept was universal to most Germanic cultures for thousands of years, and apart from the directional cognates of niðr, you find descendants of this word in most modern Germanic langauges. You only need to get on a boat and sail across the North Sea to find some descendants of nið; in the Low Lands, there's a descendant of nið in the word nijd, which means something like 'malice'.
In modern Icelandic, the corresponding word is níð (meaning something like 'slander'), with corresponding derivations níður ('down') and níðingur ('scoundrel'). It's disappointing that the closest surviving descendant of Old Norse has shed any connotations of brawny Viking warriors rowing their longboats up the south fjord, but then again, Iceland is more known more these days for its volcanos, geothermal power, stinky fermented shark and collapsing banks than it is for rape and pillage.
It's equally disappointing that no widely-used cognate of nið survives in Sweden, Norway or Denmark, despite the fact that you can't swing a dead cat in continental Scandinavia without hitting the bones of Bjórnulfr the Pop-Collared doing a kegstand or some runic description of Gunnar the Gassy lighting his farts in front of the whole Þing, much to Óðinn the Allfather's great delight. Clockmaker informs me that Swedish has a cognate, niding, that retains the dirty rotten scoundrel connotations of its root, but this word hasn't been in common usage since the 1920s.
But the most telling of the surviving cognates of nið is found in the modern German Neid, which means 'envy'. The corresponding verb is sich beneiden, a reflexive. These days, if you beneid yourself, you aren't somebody's bitch. But Germanic culture being basically based around the drinking matches of a bunch of loud and proud motherfuckers with meadhorns and fur capes, to envy someone was apparently the same as lowering yourself to another person's level by desiring what he had and you didn't.
And that just wasn't cool, bro.
Sources
Jochens, Jenny. 1991. The Illicit Love Visit: An Archaeology of Old Norse Sexuality.
Sørenson, Preben M. 1983. The Unmanly Man: Concepts of Sexual
Defamation in Early Northern Society.
<small>Thanks to Clockmaker for correcting some mistaken notions had about the meanings of nið and ergi, and for providing the modern Swedish cognate for the former.</small>