1.) When a navy guy gets his girl pregnant right before a long deployment, to ensure that she'll be his once he gets back. Sometimes this is done to make it so he doesn't have to help her through the whole ordeal of being pregnant.
2.) Also refers to the act of proposing to a girlfriend prior to a long deployment, in attempt to make her feel obligated to him. This especially refers to instances where the couple is mismatched.
L:Hey, did you hear that Jen is pregnant?
C:Yea, Midshipman Quincy totally applied the Navy boxlock.
L: That sucks cuz she totally deserves better.
L: Midshipman Quincy just asked Jen to marry him dude.
C: Damn, I can't believe he wasted the Navy boxlock on her slutty ass.
L: Well, he was thinkin she'll stay faithful if he did that, but we all know she's been banging Nick behind her back anyway.