In ENM, the term nesting partner simply refers to a partner with which one shares their living space - and life, in general - as opposed to other polyamorous connections. This doesn’t necessarily mean they are the primary partner for an individual.
According to Leanne Yau of Poly Philia, the term may originate from the 1961 book Stranger in a Strange Land.
See also:
[0] Yes, Living With a ‘Nesting Partner’ Is Way Different From Having a Roommate