Okay, had to track it down. It’s from the Reeve’s Tale, and it’s a description of a 20yo young woman:
This wenche thikke and wel y-growen was,
With camuse nose and yën greye as glas;
With buttokes brode and brestes rounde and hye,
But right fair was hir heer, I wol nat lye.
In modern English (had to look up “camuse”, so that’s as good as my source, but I know the rest)
This wench was thick and well-grown
With a pug nose and eyes grey as glass;
With buttocks broad and breasts round and high,
But right fair was her hair, I will not lie.
The fact that Chaucer had “big butt” and “I will not lie” within two lines of each other is causing me disproportionate amusement. Also the fact that “this wenche thikke” works equally well in Middle English and in modern slang.