Green’s Dictionary of Slang

1. £1 million.

1978		W. Smith Hungry as the Sea in DSUE (1984).
1984		Partridge DSUE (8th edn) 76/1/: since (?) 1950.

2. (drugs) morphine.

1959		J.E. Schmidt Narcotics Lingo and Lore.
1970		T. Southern Blue Movie (1974) 188: Every two hours or so, just as Les was about to come round, he would hit him with five grains of the Big M.

3. marriage.

1966

Current Sl. I:3 1/2: The big M, n. Marriage.

4. Memphis, Tennessee; Miami, Florida.

1975

L. Dills CB Slanguage 11: Big M: Memphis Tennessee; Miami, Florida.