1. [mid-19C–1950s] a sleep.
2. [late 19C–1900s] (US) a refusal or collapse [note lay down v. (1)].
3. [1900s–30s] a place to sleep; also attrib.
4. [1920s–30s] (US drugs) the price of admission to enter and smoke in an opium den; thus an opium den [lay down v. (2)].
5. [1930s+] (UK Und.) a period of remand in prison.
6. [1930s+] (US) a certainty [one can lay down money on it].
7. [1950s] a burial.
an absolute certainty.