Douglas - It's less of a catchphrase, more a case of, to the recipient, and they vary -
You're off on another of your nasty pointless needling thread-derailing fight-picking expeditions, and I'm shutting you down before you even get started.
That's what it is, now you and anyone else who follows that sad pattern, knows what it means.
Makes for nice tidy on course threads, saves me getting wound up, and frustrates the nasty pointless small minded juvenile culprits.
A result all round.