jump on a/the grenade

1. Literally, to cover a live hand grenade with one's body in order to save the lives of others. The marine, who miraculously survived jumping on a grenade to save the members of her squad, is being awarded the Medal of Honor today.

2. By extension, to accept or put oneself in the way of some misfortune, difficulty, blame, or danger as a means of protecting someone else. I don't know why you always feel like you have to jump on the grenade for your bosses when they screw up. It's not like they ever reward your loyalty. A job in upper management brings a lot of benefits, but be aware that you'll be expected to jump on a grenade if anything ever threatens the company.