Use the adjective nth to mean "utmost" or "most recent in a very long series," like when you have to ask your brother for the nth time to please stop borrowing money from your piggy bank.
Mathematicians use this term to represent an indefinite number: "Here's a formula for finding nth roots." In everyday speech, nth satisfies a similar need — it stands for an unspecific large number. So you might complain that it's the nth time this month you've missed the bus, or that a popular TV show is stupid to the nth degree. Nth has been used in this way since the late 1800s, from n as an abbreviation of number.