post office, n. — Green’s Dictionary of Slang

post office n.

(US Und.) one who receives or delivers letters to criminals.

1935

A.J. Pollock Und. Speaks n.p.: Post office, a person who receives or delivers letters to crooks.

there’s a letter in the post office (US)

1. a phr. used to warn a man his fly is undone or his shirt is out; also used to a woman when her slip is showing.

1855		‘Q.K. Philander Doesticks’ [Mortimer Neal Thomson] Doesticks, What He Says 124: Everything looked so grandly gingerbreadly that I hesitated about going in. Little boy in the corner (barefooted, with a letter in the post office) told us to ‘go in’ and called us ‘lemons’.
1907		J.W. Carr ‘Word-List from Hampstead, N.H.’ in DN III iii 192: letter in the post-office, phr. as interj. Shirt-tail visible in a hole in the seat of a boy’s breeches.
1912		H.A. White ‘A Word-List From Central New York’ in DN III:viii 568: Letter in the post office, phr. Used by a boy to inform another that part of his shirt shows through a hole in the pants.

2. said of a woman who is menstruating.

1890–1904