Thank you so much @Kazzeon!
And it seems you are perfectly spot on again!!
I just asked the resident expert about this sentence, ムチャ言うなよ (it’s Saturday and she’s free at the moment), and she said it’s always むちゃ, not むちゃくちゃ, and it’s a common response to being asked to do something! eg. if she asks me to clean the flat, I can respond with this sentence.
So I said “great, I’ll use it at work” and she said "NO!!, you’ll get into trouble!!” and I got a stern warning to use the polite form "むちゃ言わないでください”!