Photobomb
What does photobomb mean?
To pose in the background of a picture being taken
Example
"We had one picture where we're all smiling but Jack had to go and photobomb it!"

Benedict Cumberbatch photobombing U2 at the 86th Academy Awards
Related Slang
Usage
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Last Updated: April 4, 2016
An action popular among younger people; commonly involves a nice picture being taken of the subjects but somebody slips in the background with a goofy or creepy facial expression or awkward body pose.