Sam/Eileen fanworks pair the characters Sam Winchester and Eileen Leahy in a romantic or sexual relationship. The pairing is sometimes referred to by the portmanteau Saileen.
Fans spotted a connection between Sam and Eileen when they first meet while on a banshee hunt in 11.11 Into the Mystic.
In 15.07 Last Call, following her resurrection, Sam and Eileen spend more time together, including a night of many margaritas. During a session of research (not a euphemism), they make moves towards a possible kiss when they are interrupted by Castiel's return. Dean also implies that he expects them to need some alone time, telling Sam to leave a sock on the door if need be though Sam insists that its not like that.