My thoughts were first name = n and ab able seaman. but that still leaves old man. I have heard and one for his nob used when people are scoring playing cribbage. So what is right?.
OB =Old Boy ---First name =N (first letter of name )
Nob= a Jack in Cribbage If you turn up a Jack of the same suit they call it one for his nob .
That's how I see it anyway .