To goof means to make a silly mistake or to mess something up in a minor way. When you goof on a math problem, you might write down the wrong answer because you weren't paying attention, not because you didn't understand how to solve it. When an actor goofs during a play, they might forget a line or trip on stage. These mistakes usually aren't serious: they're the kind of slip-ups that make people smile rather than get upset.

The word has a lighthearted feeling to it. If someone says “I really goofed,” they're admitting their mistake in a casual way. Compare this to more serious words like “blunder” or “bungle.” A goof is forgetting to write your name on your homework, not accidentally deleting all your files.

Goof can also mean to waste time being silly or playful instead of working. When you goof around or goof off, you're not focused on what you're supposed to be doing. Kids might goof around during recess, which is fine, but goofing off during class usually gets them in trouble.

A goof (as a noun) is the mistake itself: “That spelling error was a silly goof.” It can also mean a foolish or silly person, though this usage is less common and can sound insulting.