Mimi replies to Felix:

>> 'Bona nox, you silly ox; bona notte, dear Lotte (i.e. Charlotte); bonne nuit
>> - fie, fie! Good night, good night, we've got a long journey ahead of us,
>> gute Nacht, gute Nacht, it's high time (that we were asleep); good night,
>> sleep tight and stay fat and healthy!'
>
>I think what made some of the catch and round lyrics a little risque was
>the juxtaposition of certain syllables with others.

This is a favorite trick of Mozart and other catch writers.  I don't know
"Bona nox," but "O du eselhafter Martin" has a (relatively) blameless text
if you sing it as a single line.  When all parts come in, you definitely
hear "Lech' Arsch" due to the fact that different parts have the proper
syllables.

The subtlest use of this technique I've come across is the Elizabethan
Thomas Morley's "Fire, fire, my heart," sung in beginning US madrigal
groups from Bangor, Maine, to Honolulu, Hawaii, and beyond.  The text
runs

   "Fire, fire, my heart!
   Fa la la
   O heaven, alas, O hell,
   Ah me,
   I sit and sigh me.
   I call for help, alas, but none come nigh me.
   Fa la la"

What could possibly be risque about that? It turns out that at one point,
*and one point only*, the strong "F" of a "Fa la la" in one part is joined
to the weak aspirant of "heart" in another, and the combo creates the word
"fart." This, of course, gives a double meaning to the text.

Steve Schwartz