Thickset means heavily and solidly built. A thickset man, thickset and strong, thickset shoulders.
It describes a build rather than a weight, so a thickset person is broad and solid rather than fat. Of a hedge it means planted close, which is the older sense of the two.
1adjectivethicksetHeavily and solidly built.
“The bouncer was a short, thickset man with a neck as wide as his head.”
thickset is a build.
Thick plus set, from set in the sense of built or planted.
From thick + set.
Etymology from Wiktionary, CC BY-SA