"The job ain’t that glamorous. I mean, I spend most days making coffee, unjinxing the john."
-- Queenie Goldstein (WFT)
A lowly task involving the bathroom facilities at MACUSA, according to Queenie Goldstein who worked as a secretary (WFT).
"John" is American slang for a toilet, possibly from the British/Irish slang "Jakes," referring to the original flush toilet known as an "Ajax," invented by John Harington. Improvements to the toilet were made by Thomas Crapper. American soldiers stationed in Britain during World War I saw the name Crapper on the toilets and coined the phrase "go to the crapper." (Source: Wikipedia)
It's possible that "unjinxing the john" had to do with reversing some type of mischief that had clogged the toilets, or perhaps it is just a reference to removing any type of blockage. Unlike Arthur Weasley's investigations into wizards who had caused Muggle toilets to explode, Queenie's job was more of a menial chore delegated to a low-level employee.