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.'"
POEHLER AS CLINTON: "'Harpy,' 'shrew' and 'boner shrinker.'"
by ChiGal September 21, 2008