go and come straight back as fast as you can
Vá num pé e volte no outro, o jantar já vai ficar pronto.
Go and come straight back — dinner's almost ready.
The image is transparent: you're in such a hurry that you barely set down one foot before you're back with the other — there and back in the blink of an eye.
Falando's Idiom Trainer teaches you to recognize idiom meanings and create sentences using them, so you can speak like a Brazilian.