Green’s Dictionary of Slang

(US) Baltimore, Maryland.

1851		Debates and proceedings of the Maryland reform convention 13/2: Mr. Sherwood, of Balto. city, was in favor of the motion of the gentleman from Baltimore county (Mr. Howard,) to elect the State’s Attorneys when the Governor was elected.
1864		Moulton letter in Drickamer Fort Lyon to Harper’s Ferry (1987) 183: A report reaches us this morning that the express train from Balto. was stopped and robbed.
1914		Mencken letter 23 Mar. in Riggio Dreiser-Mencken Letters I (1986) 136: Grasty, publisher of The Balto.
1935		M. Fulcher ‘Believe Me’ in Afro-American (Baltimore, MD) 9 Mar. 12/7: Some of the gorgeous hussies [i.e. ‘pansies’] are from Balto and NY and do a thriving biz.
1976		L. Adcock Not for Truckers Only 45: A Tale of Two Cottonpickers [...] Stepping Out to Balto [HDAS].