Any situation that causes you to lose your boner very rapidly.
1. When you are masturbating and your parents walk in giving you a "Boner-Shrinker".
2. Waking up in the middle of a sexy dream with a hard-on next to your FAT UGLY companion lover giving you a "Boner-Shrinker".
3. When you are at the beach with a bunch of hot chicks and you have a boner. Then you get in the ice cold water.
4. Hilary Clinton would also be considered a "Boner-Shrinker"
A phrase used by men to describe something or someone so ugly it "shrinks their boner" or immediately halts an erection.
"Awww man, I saw my grandma naked last night, total boner-shrinker."