The best explanation for this I could find is at The Straight Dope:
Why do outhouse doors have half-moons on them?
Quote: "The idea that outhouses always have moons on them has been
perpetuated largely by several generations of cartoonists (e.g., Al
Capp), probably none of whom ever saw one either...The woodshed was
often a lean-to attached to the schoolhouse, but the most accepted
arrangement was to place it between the schoolhouse and the privy,
with a fence separating the boys' entrance from the girls'. The
ancient designation of privy doors was to saw into them a sun (for
boys' toilet) and a moon (for girls' toilet)....As time passed by and
frontiers were pushed further westward, the gentlemen's restrooms fell
into disrepair and eventually were abandoned altogether.
Accommodations for ladies were better maintained and this is why the
moon symbol remains on many outhouse doors today. Its original
meaning, however, was lost to the general population sometime in the
Also, from The Outhouses of America Tour:
Why are there moons on Outhouse doors?
Quote: "Probably the most recognizable symbol associated symbol with
the traditional outhouse building is the familiar crescent moon carved
into the privy door. Actually, the symbol is an ancient one, and was a
sign for womanhood in colonial days and on the frontier."
I suggest you read the complete articles for more interesting details.
I hope this helps!
Search Strategy (on Google):
"half moon" outhouses