Betty tells Simon they once thought an elf names Hugo had tried to "pull a blue flu" to get out of work but it turned out he'd just eaten too many M&M's (blue ones). A "blue flu" is a term for when a police union goes on an unofficial strike by having the officers call in sick. Since the elves are not police officers and it was established moments earlier that the elves don't even know what a union is, the use of the term here makes no sense.