What does "dressed to the nines" mean — and why is it funny?

informal

Meaning

Dressed very smartly and elaborately, in your finest clothes.

Where it comes from

An old British phrase. Its true source is debated, with 'to the nines' being an old way of saying to perfection or to the highest possible degree.

Why it is funny

The gentle humor is the mystery number. We all happily say it, complimenting someone's outfit with a precise-sounding 'nine' that nobody can explain or count.

Used in a sentence

"They turned up to the party dressed to the nines."