A common catch phrase used in the popular classic cartoon "Pepe Le Pew" of Looney Tunes. The cartoon would feature a skunk inevitably falling for a cat who always seemed to get a strip of white pants on her otherwise completely black body...giving said feline the appearance of a female skunk. Pepe would hop around (literally hopping) trying to woo the extremely hesitant (understatement) kitty.
The phrase "le sigh" was often followed with a "le purr" in the series.
Many little blogging junkies have taken this saying and put it to their own usage to describe the feeling of dejection, apathy, dreaminess, etc. Sadly, most do not know the origin of the phrase. Even so, it is still a quite useful and popular term.
Sad Blogger: "Le sigh, I got dumped today."
Apathetic Blogger: "Le sigh, I am so bored."
Dreamy Blogger: "Le sigh, So-and-So is so sweet."