feces
"The train will operate in doo mode while passengers disembark at the station."
Initialism of driver-only operation.
In plain English: Doo is an old-fashioned slang word for doing something silly or foolish.
"The doo in my pocket is a small bag used for carrying money during traditional dances."
Used as a scat word in song lyrics.
"The jazz singer improvised the chorus by replacing the melody with rhythmic doo sounds instead of words."
The word doo emerged in English during the 1950s as a term used by children when speaking to one another. It entered general usage through this informal, childish speech pattern rather than originating with an older meaning.