I have seen many people add that a child has Never Married beside their name.
That abbreviation is clearly made by someone who created their tree and it was meant for them to use. Through ‘Smart Matches’, I suspect people that simply copy other people’s info down without looking at things simply propagated the info. I think to your particular tree it wouldn’t need to be there. If you wanted, however, you could make a note that it was there, until you figure it out. But I suspect it means Never Married, as I see it in Canadian trees all the time- even for children.