As suits a legendary song like this one, there are about five legends of its genesis on the web.

A good legend must have a point in itself. In this case the line must be:
1) a messenger says someone "regrets she is unable to lunch today",
2) CP stumbles over the logic behind it; she must have meant "... with you",
3) since if she were unable to lunch at all, she must be very ill indeed, which she would have written (or told the messenger) first, being the more important news.
4) CP thus realises that the phrase can be taken as a breach of the law "important things first", a fruitful source of comic effects. -

In my opinion this is one of the best songs ever in the category of black humour (with a "u", putting to shame many British specialists). A good idea executed with perfect craftsmanship in poetry and music.

The lyrics are a parody in the older sense of the word (literally: besides the path): two normally seperated worlds are being mixed.