Inflammation of the bursa (a small fluid-filled sac) located near the Achilles tendon in the heel, causing pain and swelling.
From Achilles (the tendon of the heel, named after the mythological hero) plus bursa (Latin for pouch or sac) plus -itis (suffix meaning inflammation).
The Achilles tendon is named after the hero precisely because it's a tendon 'strong enough to carry a warrior'—but it's also the most common tendon to rupture in sports injuries, and bursitis around it is a classic runner's injury.
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