Green’s Dictionary of Slang

1. (US) used attrib. to signify size/weight/number.

1944		E. Wilson 16 Mar. [synd. col.] I’m not throwing these box-car figures [i.e. of a nightclub show costing $20,000 to produce and $8000/week wages] around because I have any familiarity with such money.
2002		R. Gordon Can’t Be Satisfied 106: ‘Bo was a nice boy, just couldn’t read and write [...] Wouldn’t know his name in boxcar letters’.

2. (US) usu. in pl., a large foot or shoe.

1950		in DARE.
1960, 1975		Wentworth & Flexner DAS.

3. a large, clumsy person.

1966–70

in DARE.

4. (US Und.) a prison punishment cell.

1977

E. Bunker Animal Factory 148: They reached the ‘boxcars’ . . . They’d begun as regular cells but then concrete blocks had been extended between each out to the walkway above. A solid door was added, and when it was closed, a man screaming inside the cell was just a squeak outside of it.

5. (US prison) a cell.

1992

Bentley & Corbett Prison Sl. 5: Boxcar A prison cell.