Two members of the same swim team start dating (and or hooking up), probably due to the large amount of time spent almost naked together.
Leads to awkwardness and potentially someone quitting the swim team when they break up.
It's a combination of the words "swimming" and "incest" because a swim team is like a huge family and most swimmers consider each other to be brothers and sisters.
Cassie and Dave have been on the same swim team for 3 years then they started dating. Totally swimcest.
Annie quit the swim team after Joe broke up with her! Swimcest only leads to trouble.