Best definition
When you are writing and make a spelling mistake or fuck up. Like a typo, but in writing.

John: This note says “i want to cum all over your body”…

Kaley: Oh yeah sorry. it was supposed to say “I want to come over.” it was a writeo.
writeo: define #2
Like a typo but made when you’re writing with a pen or pencil or marker.
I figured extension 234234234234 was a writeo when I saw it on the listing but alas, I was wrong.
writeo: define #3
Like a typo, but written.

Usually happens when you are thinking of something other than what you’re writing about. You start writing out the word and then finish it with something completely unrelated.
“Oops! Write-o! I accidentally wrote sex instead of September.”