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A loud, angry noise someone makes in a men's room stall when they hear another guy coming in. It's like saying, 'I'm here, and I'm not leaving until I'm done.'
'I'm still in here, you idiot!' he yelled through the door.
'You better not try to open this door, or I'll kick your ass out of here.'
'I'm not done yet, and I'm not letting you in!'