A woman that is ugly, bitchy, harpy, shrewish, or has any other unattractive characteristic that makes her sexually undesirable to a man. Said of a woman that temporarily kills sexual desire because of physical or personality traits that do not a man want to have her.
Joe: "Kate is a Boner Shrinker."
Alex: "Yeah, she ain't very pretty."
Joe: "Not just that, but she is a complete bitch. That is what I meant."
A pair of pajamas so unattractive (for example: baggy flannel pajamas) that they make the male penis shrivel back inside, preventing an erection from occurring.
Yo girl, I was just ready to get my freak on until you changed into those ugly ass dink shrinkers.
When referring to a man's erection as a boner, a boner shrinker would be anything that diminishes it. Most likely an unattractive woman. Made famous by the 9/13/2008 opening skit of Saturday Night Live dialog by Tiny Fey and Amy Poehler:
FEY AS PALIN: "So we ask reporters and commentators, stop using words that diminish us, like 'pretty,' 'attractive,' 'beautiful.'"
POEHLER AS CLINTON: "'Harpy,' 'shrew' and 'boner shrinker.'"