'Hart ist der Zahn der Bisamratte?' [Mature Subject]

I'd like to know the full text of the little couplet that he recites.

This is the scene where she wakes up, and climbs over him to make their morning coffee, and stops in amusement, as she feels his sexual arousal.

Sleepily, he recites a couplet that sounds like,

"Hart ist der Zahn der Bisamratte,
Doch haerter ist der Morgenjatte!"

In a version with English subtitles, they didn't translate this literally, and instead, tried to find a couplet that would convey the same schoolboy humor.

Anyway, I'd like to know the exact text, if there's a German speaker who knows it, and I'd also like to know--since this appears to be a well-known couplet in German--if this is some common joke in Southern German in the military or in the schools. I especially cannot make out the last word of the couplet...Morgenjatte is what it sounds like to me, but that can't be the word, even if it's southern slang.

=========

The English subtitles I saw made a verse using "King Kong" and "dong" as slang for penis. The actual German verse seems to mean something like "The tooth of the muskrat is hard, but harder still is the morning erection!"