Green’s Dictionary of Slang

(US prison) of sentences, served consecutively.

1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 215: stacked, adj. – concurrent terms.
1992		Bentley & Corbett Prison Sl. 23: Running Wild also Stacked [...] When a person receives multiple prison sentences that must be served consecutively.