Combination of "braking" (i.
e., related to a
car's brakes) and "breaking in period," describing the time it takes to adjust your foot to the sensitivity of the brakes of a
car that you don't regularly drive, such as a friend's
car, your spouse's
car, or a rental car. At first, the brakes are either too sensitive, which causes you to slam down the breaks and jerk to a stop, or, too firm and require you to push down harder with your foot than you are used to.