Brexit means Brexit
Something you say if you are pursuing a hopelessly vague, potentially catastrophic exercise in national self-harm driven by misconceived ideology or the bone-headed illusion that you somehow still rule the waves (you know, the way King Cnut thought he did); you have just found out that you won’t be able to rely on ancient residual prerogative powers to stab yourself in the eye after all, and you want to brush off uncomfortable questions about what on Earth you think you’re playing at.
Also, what a thoughtful lawyer says in a legal opinion.