Rejoice, Delighted, Pleased
If there is a drum in your mouth it's time to rejoice! And because of your mouth drum your rejoicing will be loud and booming!
Imagine having a drum in your mouth and rejoicing about it! Imagine how loud it would be!
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You loudly rejoice for a while when suddenly someone runs over and shoves a key (き) in your mouth, locking it shut. You can't rejoice anymore! You need that key to unlock your mouth!
Imagine if people could just stop the rejoicing of others by locking their mouths shut with keys. What a sad world.