Noun. Literally, A Boner Kill is any thing, person, event, or disruption of the 5 senses, that causes one to lose an erection. Boner Kill is also used to refer to any situation which takes a dramatic turn for the worse.
"Dude, last night, I was over at Cheryl's and we were hookin up, everything was normal and good, when outa no where she tells me that she loves me. What a Boner Kill, right?"
friend - "dude it looks like someone hit your car"
friend 2 - "Talk about a Boner Kill"