(Britain,idiomatic) To commit a faux pas, especially in speech, such as by tactlessly speaking of a subject that should not be mentioned.
1978, Ambroise Vollard, Violet M. MacDonald (translator), Recollections of a Picture Dealer, 2003, Dover Publications, page 28,
It was on one of those Tuesdays that I dropped a brick, the memory of which still haunts me after more than forty years. A very loquacious lady, whom I had never seen before, appeared to be talking through her nose. “Don't you think,” I said to one of my neighbours, by way of starting conversation, “that lady would be well advised to sound her trumpet less often?”
“You bet I do! I've not been able to get used to it these thirty years.”
And as I stared at him, horror-struck:
“Yes. I'm her husband.”
1998, Harry Brewster, A Cosmopolite's Journey: Episodes from a Life, page 216,
I remember dropping a brick when I first met her. As she at once started calling me 'Harry' in the way Americans are apt to do, the surname becoming immediately superfluous. I ventured to ask her what her Christian name was.