“One for his Nobs” for a person holding the Jack of the same suit as the cut card. It is scored one point when counting your hand or your crib.
“One for his nibs” or One for his heels” is two points for the person who’s cut is a Jack scored immediately.
Along with an earlier form “nabs,” “nibs” was based on “nob,” an alternate spelling of “knob” and an 18th century slang term for “head.” The “head” in question was both literally the human head and “head man,” or an important person (THE JACK)
Most people just refer to either as nibs nowadays.