fall 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 falling on a grenade to save the members of her squad, is being awarded the Medal of Honor today. At first, you don't think you could fall on a grenade for complete strangers, but that all changes once you're on the battlefield together. The soldier who saved her entire unit by falling on the grenade is going to be recognized posthumously for her bravery.

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 fall 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 fall on a grenade if anything ever threatens the company. I know the expectation is that I'll just fall on the grenade for the good of the department, but no! I will not take the blame for something I did not do!