I suppose your native language is Dutch, so, literally, cazzo means lul.
However, there's no literal meaning in the rhyming "verse", supposedly "by the poet".
It's just an intentionally rude rhyming distich to salaciously comment on the tenente's boastful (or appeasing) statement.
The prostitute does not really believe that it will be an easy task, so she's ironically saying "clench your buttocks and it'll be a fucking goodnight...", but I'm sure she's grinning and meaning the exact opposite. In fact I translated the piece you don't understand, but it's really the rhyming between pazzo and cazzo that counts.
In fact one might substitute WTF to "buonanotte al cazzo" and the result would be equivalent.
As a general rule "(del) cazzo" is a good equivalent (in depreciation) of "fucking".
I wish you're better off now. Maurizio /Maurits