cailler

Literally to curdle (le lait a caillé), but in everyday spoken French, se cailler (or the impersonal ça caille) is extremely common slang for being cold — on se les caille dehors. The slang sense is far more common in conversation than the literal dairy meaning.

• Le lait a caillé parce qu'il était périmé. — The milk curdled because it was expired.
• Ça caille dehors, prends un manteau ! — It's freezing out there, grab a coat!
• On s'est caillés en attendant le bus sous la pluie. — We froze waiting for the bus in the rain.

Se cailler (to be freezing cold) is casual slang, not appropriate in formal writing — use avoir froid or il fait froid in any professional or written context.

• expression "Ça caille" — The slang phrase for 'it's freezing' plays on cailler's literal sense of milk curdling/solidifying in the cold; the writer Louis-Ferdinand Céline is credited with an early written use of 'se cailler' to mean 'to be cold,' around 1930.