(n.) a vulgar, uncouth young man
A clinchpoop, as one particular sixteenth-century guide to gentlemanly breeding advises, is a man who has “in him any humble behaviour,” and “knoweth no fashions.” In other words (and not in the words of sixteenth century “roisterers”), a clinchpoop is a wholly uncouth, uncultured, or unfashionable man.
But why clinchpoop? Etymologically, this is something of a mystery.
The Oxford English Dictionary suggests that this word might be derived from ‘clinchers’—workers in boatyards who would once have been tasked with fastening ‘clinching’ together the bolts on ships. (That might make the ‘poop’ that turns up here a reference to the ‘poop’ or stern of a vessel—that is, the same as in poop deck.)
If that’s the case, we can presume that these manual labourers weren’t quite so well known for the gentlemanly conduct or their knowledge of the finer things in life, and it’s from there that the uncouth clinchpoop clinched itself a place in the dictionary.