"Niddering" is an old-fashioned word for a
coward or
honorless
wretch. Originally a noun, but since it looks like a
participle, it gets used as an adjective sometimes. It's interesting because it's a mistake that solidified in the language -- the word in
Old English was "
nithing," derived from an
Old Norse word for "
scorn." Naming someone a "nithing" was a public condemnation for life under Anglo-Saxon and early Norman kings.
A 1596 copy of William of Malmesbury's twelfth-century history apparently misread the original quite badly, rendering the word as "niddering." "Nithing" also has become "niding" in modern English which is slightly more explainable, since the character used in Old English for one type of "th" sound looks to modern eyes like a lowercase "d" with a crossbar near the top (lowercase eth, ð or ð in browsers that will show it). Or maybe the sound just changed over the years. How another syllable got in there is difficult to figure out, but the mistake has influenced the language. The OED records three uses in the 17th century, one in 1706, and then none for a while, until the nine uses during the 19th century when historical fiction gained greater popularity.
liveforever tells me that 'The word is still in use in Danish, albeit as an archaicism (hasn't been current since the 19th century): "nidding".' That's with a pronunciation most likely close to the OE/ON original, with the double d said like '"dd" in Welsh. It equates to a very soft "th".'
Sources:
http://wordsmith.org/awad/archives/0602
http://www.bartleby.com/61/89/N0098900.html
http://www.squirrel.com/squirrel/asatru/nithing.html
http://www.xrefer.com/entry/363033
http://dictionary.oed.com