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While this is not a particularly tricky or advanced phrase, it’s still very handy in situations when, say, you are with other people and you want to excuse yourself and go to the loo, for instance, but you don’t want to just disappear without a word, nor do you want to specify where you’re going. This is when the phrase ‘I’ll be right back‘ comes in very handy.
There are also some variations of the phrase – I’ll be right in / up / down / etc. – that you can explore here.
Confessions of the Linguistic Spy 