Christian Combs isn't taking 50 Cent's recent online digs lying down ... he's hitting back at the "senior citizen" by bringing up his baby mama!

Here's the deal ... 50 Cent shared a clip of Christian trying to get a "Bad Boy" chant started at a recent concert -- which went over like a lead balloon and made for a hilariously awkward moment.

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50 captioned the post, "Damn 👀his timing was just a little off. 😳" ... which got fans cackling online. Christian, however, wasn't snickering -- he was crafting his own roast online.

Combs responded in the comments, "The real reason why this senior citizen Curtis is angry and keeps posting me is cuz his BM 😂."

While Christian didn't provide any clarity on the comment, it seems to be a reference to Daphne Joy ... the mother of 50's 13-year-old son Sire, who dated Diddy in recent years.

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Basically, Combs went at 50's personal life -- though the rapper quickly brushed it off -- and flamed the new Diddy documentary Christian and his brother Justin Combs posted about.

As you know ... 50 regularly posted about Diddy during his federal sex trafficking trial -- which ended with his rival getting a guilty verdict on 2 of the 5 charges against him. He was sentenced to 50 months in prison.

We reached out to 50's rep ... so far no word back.