Green’s Dictionary of Slang

lay down! excl.

(US) be quiet! shut up!

[US]T.A. Dorgan in Zwilling TAD Lex. (1993) 53: (OS: Talking about your boys progress in school now that the world’s series is coming on) Aw lay down Sisler will lead both leagues.
[US]Ersine Und. and Prison Sl.
[US]Goldin et al. DAUL 122/2: Lay down! (P) Shut up! Go to sleep (This is one of the most common expresions [sic] in prison slang.).