i believe it stands for nonmetallic metal. it’s when you paint armor/weapons etc. to look like metal but without using metallic paints. For example, you could paint gold by using a combination of browns and yellows. It’s a very effective and clean technique but quite challenging to master
but i used mithril silver ! its about as metallic as you can get ! a combination of dry brushing the black armour all over then blocking it out with black to leave just the edges looking slightly worn - then adding scratch marks with small bits of m.silver
so heres some more pics - still not quite finished painting yet trying to neaten up beards and put final touchs here and there i have b4 and after pics - 1st unit of warriors w/ hand weapons