With bulging biceps that can crush walnuts (and more than likely the can they came in), Madonna isn't someone you want to tick off. But the London Daily Mirror says a worker on Guy Ritchie's new movie did just that by talking a mite too loudly -- and too freely -- about one of her kids.

It seems a couple of female crew members were chatting on the London set of the unfortunately titled "RocknRolla" last Friday when one of them asked the name of the Ritchies' 2-year-old son David, whom the Big M was showing around the studio that day.

The paper says Madonna, who is still in the process of adopting the Malawi-born tot, heard one of the workers jokingly reply, "Lucky B****rd," a quip that made her go maternal on the staffer's a**.

"Madonna looked astonished by what was said and words were exchanged between them," relates an eyewitness. "Madge seemed very protective of her son and didn't want anyone talking of him in such a manner."

Continues the source, "At the end of the day, she's just a mum and doesn't want anyone speaking ill of her family. She looked really hurt and shocked by the comment and let her feelings be made known."

The put-out pop icon refrained from popping the crew member's head like a grape, instead deciding on a more traditional move: The paper says the gum-flapping worker was canned.

"It's Guy's film and Madonna is his wife, so they're very much the top dogs," points out the snitch. "It was a bit foolish to speak like that, to say the least, so it's no surprise it all kicked off."

Madonna's rep issued a "no comment" on the alleged verbal smackdown.

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