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別腹 betsubara
Dessert has its own stomach.
What does betsubara mean?
The scientific-sounding excuse for ordering cake after a full meal. Officially there is a separate compartment reserved for sweet things.
Example
お腹いっぱいだけどデザートは別腹。
I'm stuffed, but dessert goes in a different stomach.
Vibe
Who you can say this to, and how it lands.
Safe to use Fine in ordinary conversation.
The story behind betsubara
The joke is anatomical. Betsu, separate, plus hara, stomach: a second compartment that stays sealed during dinner and opens the instant a dessert menu appears. Sweets have their own stomach is one of those phrases everybody says with a straight face while ordering the parfait.
The funny part is that the body half backs it up. Show a full stomach something it really wants and it can relax to make room, which television shows here have demonstrated with great glee. So the excuse is doing better than most excuses. It is also flexible: friends will claim betsubara for a second bowl of ramen, and the word is even borrowed for a budget you were absolutely not going to spend.