Refers to a result of plus 2 to par (or worse ) on a golf hole (aka double bogey, triple bogey, etc.). Also, a player who has 3 or more putts on a hole has committed a Timmy.
Origin - Timmy is a guy that bailed on a golf commitment in the 11th hour. A scoundrel.
Singular - "Another Timmy. Today just isn't my day on the links."
Plural - "Dammit, I had a lot of Timmys today. My game is in the tank."