Green’s Dictionary of Slang

altar n.

also altar room, alter

the lavatory.

[US] ‘Jargon of the Und.’ in DN V 437: Altar, The stool in a toilet.
[US]Irwin Amer. Tramp and Und. Sl. 18: Altar. — A toilet, more especially the porcelain bowl.
[US]Mencken Amer. Lang. (4th edn) 577: The substitution of far-fetched figures for literal description gives the felon altar for toilet-seat.
[US]Monteleone Criminal Sl. (rev. edn) 11: alter A toilet seat. alter room A toilet; a privy; a rest room.
[US]Goldin et al. DAUL 18/1: Altar. Toilet flush-bowl; a bucket or any similar substitute.
[US]Ragen & Finston World’s Toughest Prison 789: altar – A toilet, especially the porcelain bowl.
[US]R.O. Scott Gay Sl. Dict. [Internet] altar room: Syn: for toilet.

In phrases

worship at the white altar (v.)

to vomit.

[US]J. Stahl I, Fatty 171: Virginia, on hands and knees, worshiping at the white altar.