When you interject, you interrupt or put yourself in the middle of something. Ever have a coworker interject himself into the plans for your big project at work when he wasn't really needed?
First used in the late 16th century, the verb interject comes from the Latin word interjicere, which combines inter-, meaning "between," and jacere, meaning "to throw." It's most often used to describe something that's abruptly thrown into a conversion. It can be considered rude to suddenly interject a question into a conversation that doesn't involve you. But you might interject a personal story into a boring presentation to liven things up.