flying baker, n. — Green’s Dictionary of Slang

flying baker n.

[naut. jargon baker = B; the flag signifying the second letter of the alphabet is red; it signifies ‘Beware, keep off’]

menstruation.

1948		Word 4 183: Indirect allusion to red or blood also is made: the gal’s at the stockyards, [...] the flag is up, and flying baker. This last is common Navy parlance [...] In the Navy alphabet . . the B or baker flag is red, and when it is up it may mean . . ‘Beware, keep off.’.
1970		in DARE.
1999		J. Randall ‘A Visit from Aunt Rose’ in Verbatim XXV:1 Winter 25: Codes that refer to blood include the red flag is up (sometimes shortened to just the flag is up or the flag is flying; also sometimes flying Baker, since Baker is the Navy code for B, and the B flag is red).