Green’s Dictionary of Slang

(US black) used as a term of quantity, esp. in the context of the length of a prison sentence; thus a prison sentence.

1969		‘Iceberg Slim’ Pimp 290: A pimp ain’t got the heart to do a slat down there.
1979		‘Iceberg Slim’ Airtight Willie and Me 73: I caught a slat in the joint on the coal pile.