A horse that is very well trained and calm, that isn't easily spooked. Term originated for Military horses who must be "bomb-proof" so they don't spook at the sound of a bomb/gun/etc.
Children's riding ponies often must be bomb-proof because kids tend to run and jump and make loud noises--all things that might scare an ordinary horse and cause him to hurt somebody.
My old mare Goldie was a bomb-proof horse-- We could drive the tractor right past her and she wouldn't even flinch!
Someone who jumps from one relationship immediately into another one.
Serial monogamists can not stand to be alone and often suffer from vast commitment and insecurity issues.
Because they jump into relationships immediately after the previous one has ended, serial monogamists typically don't take the time to reflect on their behavior or why their previous relationships failed; thus, they end up making the same relationship mistakes over and over again.
Person 1: Damn, Dustin already has a new girlfriend?! It's only been two weeks since he broke up with his fiance! I think he's a sociopath.