This obscure word — meaning a coward, or cowardly — is the result of an error, but one which has been rubbed true by time. It’s very rare, but it can carry a special punch when it appears:
Stripped of its outer integuments of salacity and fraud, the inner man is revealed as timid and niddering, lying to the last firm handshake and as sickly yellow as a poisonous toadstool.
Paul Johnson, writing in the Spectator, 22 May 1999.
The historically correct form, which is now even rarer still, is nithing. The fault was that of the printer in the 1590s who had the job of setting William of Malmesbury’s historical works in type. He misread the eth character in the old spelling niðing as a d followed by a mark, which he assumed meant an e had been omitted. The result was nidering, which later writers made to conform with the usual rules of English spelling by adding a second d.
Its original, nithing, derives from an ancient Scandinavian legal term, the Oxford English Dictionary explains, that meant a person “who has committed a crime so heinous that no possible compensation may be made for it.” It was taken over into the legal system of England before the Norman Conquest in the sense of a coward or outlaw. Later, it came to mean a miser or a treacherous person. Conversely an unnithing was an honest or generous man.
Niddering owes much of what little circulation it has had in the past two centuries to the once-popular Sir Walter Scott, who used it in Ivanhoe in 1819.