When a person shows off or makes a smart-ass remark and all you can do is stare, nod, praise them, roll your eyes at them or a combination of the four. Seen frequently in BBC' Sherlock between Sherlock and John.
Sherlock Holmes: Her coat is slightly damp; she's been in heavy rain in the last few hours. No rain anywhere in London in that time. Under her coat collar is damp too; she's turned it up against the wind. She's got an umbrella in her left-hand pocket, but it's dry and unused: not just wind, strong wind, too strong to use her umbrella. We know from her suitcase that she was intending to stay overnight, so she must have come a decent distance, but she can't have traveled more than two or three hours because her coat still hasn't dried. So, where has there been heavy rain and strong wind within the radius of that travel time? Cardiff.
Watszoned Dr John Watson: That's fantastic!
Sherlock Holmes: Do you know you do that out loud?
Dr John Watson: Sorry. I'll shut up.
Sherlock Holmes: No, it's... fine.