At a driving school
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Man: Oh, oh, I like this horn.
Driving Instructor: Leave the horn alone and buckle up.
Man: I’m sorry. I won’t say another word. I’ll buckle up.
Driving Instructor: No, I mean buckle up your seat belt, your safety belt.
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What does “I’ll buckle up” mean in this dialogue?
Does it mean “I’ll buckle up my mouth”?
Thank you
Video link: (20 seconds)