Throwing someone “under the bus” is an expression used to describe the act of a colleague or manager attempting to deflect blame from themselves by foisting fault on to someone else. Your boss might throw you under the bus in a public forum, in which what he's doing is obvious, or behind your back, which you may never be aware of. Protect yourself by doing your job well and keeping an eye out for blame that inappropriately falls into your lap.
If your boss finds himself in a tenuous position with his supervisor or a major customer, he may throw you under the bus to save face by suggesting whatever the problem is, it is your fault. If you witness this exchange, you'll immediately know you were thrown under the bus. Your options are to defend yourself on the spot, or take the issue up with your boss after the fact. It's a fine line between protecting your professional reputation and backing up your boss, particularly when a client or large account is at stake.
You may never know if your boss subtly throws you under the bus. For example, if your boss goes into a board meeting unprepared for a financial presentation, rather than admit his error, he may say you didn't get it done on time. You'll never know that conversation took place unless a board member confronts you about it or it gets back to you in some other way. The only other way to detect this type of behavior is if you notice a lack of faith from other department heads or you don't get selected for key assignments or projects -- even though you're otherwise qualified.
Address the Problem
Some bosses expect employees to be “team players” and take the heat for them as part of their job responsibilities. If this is not something you're comfortable doing, have a direct conversation with your boss. If your boss regularly blames you as a way to cover up his own poor job performance, you’re best served taking the issue to human resources for mediation. A direct conversation with your boss is not likely to be effective, as his tactics are deceitful and underhanded and are clearly intentional in nature.
Steer Clear of the Bus
A boss who regularly throws his staffers under the bus will eventually be found out. Your best protection is to always do your job to the best of the ability so there's no chance of setting yourself up for a poor performance scenario. Keep track of instances in which your boss takes credit for your work or deflects blame on to you. This gives you a record in the event you need to take the issue to a higher authority in the company.