“To go underwater.”

To do something on the sly behind someone's back, such as breaking a rule or a diet when you think no one is looking.

Heard in the wild

مگه رژیم نداشتی؟ پس این شیرینی چیه؟ زیرآبی می‌ری؟

Aren't you on a diet? So what's this pastry? Sneaking around, are we?

More teasing and scolding

خاک تو سرت!
khâk tu saret!
“Dirt on your head!”
Shame on you, you hopeless thing. Can be a real scolding, but between family and close friends it is often half fond and half exasperated.
دیگ به دیگ می‌گه روت سیاه
dig be dig mige rut siâh
“The pot tells the pot: your face is black!”
The pot calling the kettle black: said when someone criticises a fault they share.
موش تو سوراخ نمی‌رفت، جارو به دمش می‌بست
mush tu surâkh nemiraft, jâru be domesh mibast
“The mouse couldn't fit into its hole, so it tied a broom to its tail.”
Said of someone who can't manage their own affairs but piles on more, or drags others along with them.
برو کشکتو بساب
boro kashketo besâb
“Go and grind your kashk.”
Mind your own business, go away and do something useful.