flush, v.² — Green’s Dictionary of Slang

flush v.²

[SE flush, to empty/refill a lavatory bowl]

(US) to reject, to cancel, to discard.

1955		E. Abbey Brave Cowboy (1958) 39: Flush that noise!
1958		E. Gilbert Vice Trap 33: ‘Flush him,’ I said.
1972		B. Rodgers Queens’ Vernacular 83: flush (fr teen sI, late ’50s = to evacuate toilet-wise) to reject; to cancel; to eliminate.