Not my field of study, but I know how to track down articles from sources more reputable than wikipedia. I dug one up that indicates nithing is as serious an insult as it's being made out to be. In other words, my school's JSTOR subscription, let me show you it.

ETA: I can also provide a .pdf copy of the article on request. It's actually quite interesting.

Nithing was a status term, very similar to outlaw, but carrying an even greater opprobrium with it. A person could be outlawed as a killer and yet not be a nithing. A nithing was the lowest of the low; he was a violator of trust, a truce-breaker, a betrayer of friend, kin, or guest, the murderer and more. Conviction of certain crimes carried nithing status with it automatically*; in other cases it appears that naming someone a nithing was itself a solemn juridical ceremony. The Anglo-Saxon Chronicle relates how Earl Swein Godwinesson, through deceit and treachery, slew his cousin Earl Beorn. King Edward and the whole army convened and declared Swein to be a nithing. The effect of the designation was immediate: six of Swein's eight ships deserted him, and Swein was forced to flee across the channel to Flanders.

*Here Miller cites Larson, Earliest Norwiegian Laws, which states:

In medieval Norwegian Gulathing and Frostathing laws, an outlaw forfeited all his movables but not his land; land, however, was forfeited for nithing crimes, which both the Gulathing and Frosthathing laws specifically enumerate.

Both the quotation and the footnote are on p. 186 of the cited text.

Miller, William I. (http://www-personal.umich.edu/~wimiller/cvnet.htm) "Choosing the Avenger: Some Aspects of the Bloodfeud in Medieval Iceland and England." Law and History Review 1 (1983): 159-204.