Definition - What does Hairpin mean?
Techopedia explains Hairpin
The term hairpin comes from the shape of a woman's hairpin, which doubles back over itself, creating two "legs." In telephony, a single call may be turned into a two-legged call. The two legs intersect at the destination number. The first leg is the call from the source number to the destination number, while the second is the call redirected back from the called number. The term hairpin was also later used in digital data networks to denote an existing feature in some network address translations (NATs) where more than one host is placed behind an NAT. Each host is assigned a certain external address and port by the NAT.