Frost Fluid is an invention created by Doctor Proctor in the fourth book, The Magic Fruit. It works by reacting with the acids in a person's stomach, causing a person to have a need to pee (much like a diuretic) but then, whatever surface the urine hits will freeze into ice.
Nilly uses this invention in The Magic Fruit to go undetected by burglar alarms, however he got rather pee-shy, but managed to hit it just in time.