Green’s Dictionary of Slang

(US campus) a socially inept person.

1980		Eble Campus Sl. Fall 6: social donut – person without a date.
1985		Eble Campus Sl. Oct. 10: social donut – a dull person.
1996		Indianpolis Star 18 Oct. n.p.: A social donut hole [...] is a socially inept person.