Girthiest is the superlative of girthy, meaning having the greatest circumference or thickness.
The word often appears in humorous or informal contexts describing something unusually wide or bulky: That's the girthiest burrito I've ever seen.
It can also carry sexual connotations when describing anatomy.
1. bulky — thickest, Of greatest diameter; widest (Greatest in extent); fattest (Most obese); heaviest (Having greatest mass); stoutest · heftiest · chunkiest · broadest · plumpest · roundest · portliest · tubbiest · paunchiest · rotund
2. voluminous — roomiest, Having maximum space; largest (Most big); biggest (Superlative of large)
3. robust — sturdiest, Most solidly constructed; burliest (Most heavily muscled); brawniest (Most physically powerful); huskiest (Most throaty or brawny); beefiest · meatiest · strongest · stockiest