When people act like people, the theory is in pigeon
Since the decadence of the city is temporarily gone on Sunday, people can act more like themselves and people do not have to conform to the societal rules that mark the weekdays, Fridays and Saturdays.
As to the second part of this line, ‘the theory is in pigeon’ means that social rules/obligations are laxed, perhaps meaning that it literally scatters/flies away like a pigeon. However, it can also be a a play on the less common definition of a pigeon, someone who is gullible or someone who is swindled in gambling or deceived. In other words, theory is tricked into disappearing for the day.
“Pidgin” also refers to the common language spoken between two people from different languages. Specifically where trade/business is common, people of different languages will adopt a common language adequate to complete deals or accommodate transactions.
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