What does it mean "you'll wake up some morning with your tail in a sling."?
Huh. My working vocabulary seems to have a hole where this expression should be!
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
To be in trouble, in a difficult situation.
From an article about the dangers of hiking, San Francisco Chronicle (2001). "But the truth is, you need not seek out a remote site to plant your tail in a sling. Your neighborhood park can function quite nicely for this. To concoct an adventure, all you must do is: Prepare inadequately, then bite off more than you can chew." [COCA]
not a teacher
It does seem like a euphemism for a*** in a sling BrE or a** in a sling AmE, but if so you can't wake up with it. You come back with it. You've gone out to a conflict and returned badly beaten.