To back out means to withdraw from a commitment or promise you previously made. When you back out of something, you decide not to do what you said you would do.

If you promise to help your friend with their science project on Saturday but then back out at the last minute, you're leaving them without the help they were counting on. Someone might back out of a camping trip because they get nervous, or back out of running for class president after initially saying they would.

The phrase often carries a sense of letting others down, especially when they've made plans around your participation. If three friends agree to split the cost of a birthday present but one backs out, the other two have to cover a bigger share than expected.

People back out for many reasons. Sometimes circumstances genuinely change: you get sick, or a family emergency comes up. Other times, fear or second thoughts take over. Occasionally, backing out is the right choice, like realizing you've overcommitted yourself. But doing it frequently can make others see you as unreliable.