(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? Clinch? Poop? Frankly, it’s not the most attractive image.
Well, etymologically, we’ve something of a mystery on our hands here. The OED suggests that the word might be derived from “clinchers”—namely workers in boatyards who once upon a time would have been tasked with fastening together or “clinching” the bolts on ships. That might make the “poop” mentioned here a reference to the poop or stern of a vessel (i.e. the same “poop” that turns up 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.