Analogous to our existing thick thighs tag, I suggest we start tagging thick_arms.
favgroup:6281 status:any contains about 380 example posts as a starting point, based on my interpretation of what would qualify as 'thick'.
Here's a wiki page I prepared using the thick thighs page as a basis:
Show
Arms drawn thicker than "normal".
Compare the post #4742191 with "normal" arms, and post #4419291 with "thick" arms.
May apply to either the forearms or upper-arms, or both.
See also
While thick thighs is specifically for thighs and not the lower-legs, I don't see much reason to have separate tags for thick forearms and thick upper-arms.
I believe coming up with more precise usage guidelines may be possible but would take a fair bit of work analysing typical proportions, taking perspective into account — so I'm opting to defer that until a later time, considering that most existing body-proportions tags currently have only loose definitions.
Edit: added wiki link to biceps.
thick_arms
would also be added to tag group:body parts under Appendages.
Updated