OMG OED

Nitheful: Envious, malicious, wicked; as a noun, such a person. From nithe, malice, an ancient Germanic word that can also be a verb meaning to hate or be envious of someone. Also nithe-grim: grim, cruel, savage; and nithe-iwork: an evil deed. All of these words are severely obsolete (pre-1400) but it is so sad. Some nitheful person nithing these words their power did this nithe-iwork.

What could be a better comment on the Book of Revelation than this quotation from 1350?

By þe mouþe as a lyoun bitokneþ þe manaces of þe proude Men & of þe niþeful

þ is the thorn (the OED uses a more handsome form that looks like the wynn, but it does not render well in blogspot) and ð is the eth, whose name I always have to look up. Both are "th" so it transliterates, "By the mouth as a lion betokens the menaces of proud and nitheful men." Presumably it is a reference to the description of the beast with seven heads in Rev. 13:2, "And the beast which I saw was like unto a leopard, and his feet were as the feet of a bear, and his mouth as the mouth of a lion: and the dragon gave him his power, and his seat, and great authority."

Another marvelous phrase, though I am not sure how it translates:

þar beð naddren and snaken...þe tereð and freteð þo euele swiken, þe niðfule and þe prude.
There are adders and snakes...he tears and devours when evil men, the nitheful and the proud, deceive.

swike is to deceive, betray, ensnare. The -e ending makes the adjectives plural and the -en ending on swike makes it plural subjunctive. The -eð verb ending is the familiar "-eth" of ye olde English. The translations of þe and þo are probably wrong, because there are several forms that can look like those and some can work together for a different meaning. Maybe some scholar of Old English searching for naddren and snaken will comment someday.

Fun with words: the OED sayeth,

The OE. demonstrative and definite article was thus inflected:

SING.	MASC.	FEM.	NEUT.	PLURAL.
Nom.	se, laterþe	sío, séo, later þío, þíu	þæt	þá
Acc.	þone, þæne	þá	þæt	þá
Dat.	þm,þám	þre	þm,þám	þm,þám
Gen.	þæs	þre	þæs	þára (þra)
Instr.	þý, þon		þý, þon

Imagine, you used to have to buy books to find out this sort of thing. I think I even have a Celtic grammar somewhere for just such a use. How clumsy compared to google.