Green’s Dictionary of Slang

insane, out of touch with reality.

1897		A.H. Lewis Wolfville 327: I saveys at once he’s plumb off his mental reservation; an’ when a party’s locoed that a-way it makes him hostile if you derides his little game or bucks his notions.
1906		A. Adams ‘Bad Medicine’ Cattle Brands 🌐 You’re off your reservation bigger than a wolf, when you try to run things by force.

1. (US) absent, away.

1900		Ade More Fables in Sl. (1960) 113: He bribed the Hired Girl to tell him Everything that happened while he was off the Reservation.
2001		D. Jenkins Money-Whipped Steer-Job 213: I could throw off, tank the deal, miss the cut, and be off the reservation by Friday night .

2. in fig. use, drunk.

1916		O.O. McIntyre New York Day by Day 10 Aug. [synd. col.] ‘Have you gone compltely off your nut [...] or are you off the reservation again?’.
1934		O.O. McIntyre New York Day by Day 15 May [synd. col.] And the old bucks would be ‘off the reservation’ for days.

(US) within lit. or fig. required limits.

1977

Ben-Veniste & Frampton Stonewall 364: One of the greatest concerns [...] had been to develop a cover story to explain their understanding of the hush-money payments—which Ehrlichman acknowledged were intended to keep the burglars ‘on the reservation’.