Unwritten means understood and followed although never set down. An unwritten rule, the unwritten constitution, an unwritten agreement.
Britain's constitution is called unwritten, which is not quite right: much of it is written down, just not in one document. An unwritten rule is often stronger than a written one, since breaking it costs you standing rather than a penalty.
1adjectiveunwrittenUnderstood and followed although never set down.
“There's an unwritten rule in our office that nobody touches the boss's mug.”
unwritten is a quality.
Old English unwriten, un- plus written, from writan, which first meant to scratch or carve.
Old English
unwriten
Middle English
unwriten
English, today
unwritten
Built from
From Middle English unwriten, from Old English unwriten, unġewriten (“unwritten”), equivalent to un- + written.
Etymology from Wiktionary, CC BY-SA