According to the standard etymological dictionaries,
English knife comes PGmc. *kni:fa-z. This is
derived by Nikolayev from a PIE root *g(^)ni:P- ~ *k(^)ni:P-
(where *P is either *p or
*b)
This word is found as a loanword in Basque
nabas 'plough' and Gascon (Bearnese)
naves '(large) knife' [a hapax], which is remarkable for
keeping fossilized the ancient nominative. It probably also influenced the
Iberian Romance reflexes of Latin nova:cula 'knife'
(Portughese navalha, Spanish navaja, Catalan
navalha but Aragonese noballa) in having
/a/ instead of /o/.
I also think the etymology of the Latin word, usually
derived from PIE *kseu- 'to rub, to whet' should be revised,
because: 1) it needs an intrusive -n- to be added and 2)
it doesn't account for cognacy with the Germanic and Basque
forms.
Octavià