A word derived from the expressions used in Jersey Shore such as "grenade" "bomb" and "tank". It is used to describe when an ugly ass girl starts to like you due to unintentionally giving her the wrong message by being too nice, friendly etc... A very unwanted situation indeed. The name is derived from the fact that like an actual guided missile the girl starts to follow you around, flirt with you etc.
*to le ugly girl*
Joe: (unassuming) "Hey I like your hair today!"
Caitylin: "Oh thank you ;)"
*Joe now has a guided missile*