A wall thrown up by a girl (or boy) who is actively hit on
regularly by someone - usually someone who the girl wouldn't be interested in even if they weren't happy in their relationship.
The phrase can refer either to the vibe put out by the girl, or (more often) is invoked by waving one or both arms and saying "boyfriend shield".
The defensive version of the boyfriend bomb
1. "Man, she's touchy - even when I'm just talking about MY relationships, SHE throws up the boyfriend shield and doesn't want to hear about it."
2. "I got a copy of 'Caligula' yesterday and wanted to know if you..."
"WHOA! Boyfriend shield!!!"