1920s North American slang for alcoholic beverages. The magical variety actually causes the drinker to giggle.
This term was used by then-president Seraphina in her famous retort to those who criticized MACUSA's stance on Prohibition, that the magical community could continue to drink alcohol:
However, in one of her rare light-hearted moments, President Picquery was heard to say that being a wizard in America was already hard enough. "The Gigglewater," as she famously told her Chief of Staff, "is non-negotiable." (Pm)
Newt, Tina, Queenie, and Jacob order shots of Gigglewater when they visit the Blind Pig, a speakeasy in New York City (WFT).
U.S. slang c.1920s: giggle = "a light, silly laugh" + water
This is similar to the construction "gillywater," an alcoholic beverage served in the Three Broomsticks. Gilly comes from "gillyflower" = any of several species of fragrant small flowers often carried or displayed in a small bunch.