સૌથી ઉડુ પેટ ને પાતાળ.

sauthī uḍu pēṭa nē pātāḷa.

Literally: The deepest things are the stomach and the underworld.
What it means: It is impossible to truly know what secrets a person keeps hidden deep inside their mind.

Also said as

ગુજરાતી કહેવતસંગ્રહ (Gujarati Kahevat Sangrah) (1923) · read from the scan by machine — the Gujarati itself is unchecked · English is machine-made and nobody has checked it yet

Heard it differently, or know we've got the sense wrong? Tell us — that is exactly the kind of correction this needs.