Green’s Dictionary of Slang

1. (US tramp) a municipal lodging house.

1930		‘Dean Stiff’ Milk and Honey Route 45: Any harness bull can tell you where the municipal lodging house, or the ‘munie,’ is to be found.
1935		E. Anderson Hungry Men 189: I don’t like the way the birds in these Sallies and Munies order you around.

2. (US) a municipal bond.

1973

Newsweek 13 Aug. 82: [They] mounted an expensive advertising campaign aimed at peddling ‘munis’ to the masses and last year sold $750 million worth of securities.

3. a bus conveying prisoners to prison.

1989

(con. 1960s) M. Kingston Tripmaster Monkey 8: He was a convict on a locked bus staring at the sights on the way from the county jail to San Quentin [...] Always some freak riding the Muni. And making eye contact.