Of confounded origin. The forms isnel, esnel, inel are undoubtedly related to Old Occitan isnel and Old Italian snello, all from Frankish *snel, from Proto-Germanic *snellaz (“bold, quick, lively”), related to Old High German snel (“warlike, lively, brisk”), See snell.
However, Old French ignel appears to be an assimilation of the former to Latin igneus, ignitellus (“burning, ardent”), probably through folk-etymology in the sense of "fiery". Diez admits a possible origin in Latin for ignel, but not for isnel, esnel, etc.. 
ignel m (oblique and nominative feminine singular ignele)