A phrase used when one has lost control or is showing crazy symptoms; whether that is just in a social situation or when dating someone.
If you can't stop talking about someone, lock it up!
If you are making a fool of yourself in public, lock it up!
Just generally, if you are sucking, LOCK IT UP!
"Omg, I know we went on one date, but I think he's the one"
Lock it up!