Meaning of title

The "clowns" in the title do not refer to circus clowns.
Instead, they symbolize fools, as Sondheim explained
in a 1990 interview:
I get a lot of letters over the years asking what the title means and what the song's about; I never thought it would be in any way esoteric. I wanted to use theatrical imagery in the song, because she's an actress, but it's not supposed to be a 'circus'.... It's a theater reference meaning 'if the show isn't going well, let's send in the clowns'; in other words, 'let's do the jokes.' I always want to know, when I'm writing a song, what the end is going to be, so 'Send in the Clowns' didn't settle in until I got the notion, 'Don't bother, they're here' which means that 'We are the fools.'

In a 2008 interview, Sondheim further clarified:
As I think of it now, the song could have been called 'Send in the Fools.' I knew I was writing a song in which Desirée is saying, 'aren't we foolish' or 'aren't we fools'? Well, a synonym for fools is clowns, but 'Send in the Fools' doesn't have the same ring to it.

(출처: http://en.wikipedia.org/wiki/Send_in_the_Clowns)