there is no English form genuous, OED gives the derivation as from L. ingenu-us native, inborn, free-born, having the qualities of a freeman, noble, frank (f. in- (INē) + gen-, stem of gignere to beget) + -OUS.

this use of the prefix in- was probably meant as an intensive - in any case, it's not a negation.

so, disingenuous is 'not ingenuous'.

note: occasionally the prefix dis- also acts as intensive (but not here, according to the AHD usage mavens).