* A misspelling or typo: Perhaps it was meant to be "Monday sniff," which could refer to a cold or allergy symptoms that often seem worse on Mondays.
* A specific slang term: It might be a term used in a particular community or region, but it's not common knowledge.
* A made-up phrase: It's possible someone invented this term for a specific purpose.
Could you provide more context? What were you reading or hearing when you encountered this term? Knowing the context might help me understand what it means.