1 definition by Hola Gatino!
A "practical joke" or show of bromantic affection where a guy jumps on the back of an unsuspecting guy and grabs that guy's genitals/penis (i.e. the unicorn's horn).
"Wow, I was just in the lobby and saw a drunk guy jump on another dude's back and grab his junk! He unicorn ambushed him in front of everyone!"
by Hola Gatino! October 14, 2008