-ulus according to Wiktionary:
From Proto-Italic *-elos (whence Faliscan -𐌄𐌋𐌏𐌔 (-elos)), from Proto-Indo-European *-elós, thematized from Proto-Indo-European *-lós.[1] Cognate with Proto-Germanic *-ilaz and *-ulaz, whence no longer productive English -le (as in dimple and nozzle), Dutch -el, German -el.

So the PIE for little nut would be something like *knewelós