He pointed out a wild rabbit flicking the white underpart of its tail impertinently as it scampered away with comical self-importance.
"The Song of Songs"
Hermann Sudermann
You got used to the way he gnashed his teeth and tossed his head about, but occasionally, when you were occupied with the ticklish underpart of him, he would show his dislike of the operation by catching you unawares by the slack of your breeches and throwing you out of his stall.
"Servants of the Guns"
Jeffery E. Jeffery
The exposition, however, that I have made of the errors and defects of other writers, is only an incident, or underpart, of the scheme of this treatise.
"The Grammar of English Grammars"
Goold Brown