Unwritten Rule 

A rule, usually concerning social behavior, which is known by all but spoken by none. This rule is neither official nor written down. It just is.

Examples of what an Unwritten Rule is:
You do not sit next to strangers on busses/trains even if it is full - you stand.
You do not stare at people in public.
You do not tell a girl she is fat, even if she is.
You never pass a cop even if he is doing 10 under.
You do not swear in the presence of a lady.
You do not initiate unwelcomed small talk with who you are sitting next to on a plane.
You take your hat off during the national anthem.
etc.

Unwritten Rule by TTM January 8, 2006

The Unwritten Rule of Anal 

If your girl gets shit on your dick, its her job to suck it clean.

(Hot steam anal sex ends)
Bill: Babe you got shit on my dick!
Jill: So?
Bill: The unwritten rule of anal commences.

The Unwritten Rule of Anal by wetardsareus.com April 14, 2021

unwritten blowjob rule 

The rule that a man must warn a woman when he is about to ejaculate during a blowjob, so the woman has proper time to get away from the ensuing explosion of man juices. Many men do not follow this rule, prefering to make their partners suffer the onslaught of being choked by their thick, ejaculatory fluids. A common misconception is that the majority of women enjoy being drowned in semen. Although some actually do, many do not, and instead will either allow the fluids to be released on their faces, or they will revert to the handjob position and finish the job in a more comfortable, safe area.

"Everytime Jane gives me a blowjob, she reminds me kindly of the unwritten blowjob rule and then proceeds to get a safe distance away from my cumshot. Sometimes when I am lucky, she lets the cumshot hit her in her face."

unwritten blowjob rule by rawevillivewar August 31, 2006