Green’s Dictionary of Slang

1. a crematorium, the actual cremator.

1902		O. Wister Virginian 160: Yu’ dude-inhabited hot-box! Why, yu’ coon-bossed face-towel —.
1908		Sporting Times 25 Apr. 1/4: When yer gouty rheumatism spikes yer final battery have ’em bring yer here an’ shove yer in the hot box.

2. (US) a steak .

1913		Coconino Sun (Flagstaff, AZ) 26 Dec. 2/3: Railroad Eating House Lingo [...] ‘A steak well done,’ said the last arrival. ‘A hotbox and have it smoking,’ was the information given to the cook.
1945		F.H. Hubbard Railroad Avenue 325: [as 1913].

3. a battery used for applying torture by electricity.

2001

(con. 1964–8) J. Ellroy Cold Six Thousand 389: Stanton’s got the hot box. Stanton’s got the switch. Stanton’s got the clamps on his balls.