The word is rare, and seems to share an etymology with the more common (in Anglo-Norman) nuldur. By analogy, it must be a lexicalized form of the phrase quel dur (see dur1), which literally means ‘which hand’s breadth’).
conj.
1whether:
(
c.1300; MS: s.xivin
)
En ceste manere put hom saver queldour le empic seit ptisic ou noun
ii 161.105