noun, plural so·le·i [soh-lee-ahy] /ˈsoʊ liˌaɪ/, so·le·us·es.
a muscle in the calf of the leg, behind the gastrocnemius muscle, that helps extend the foot forward.
Origin of soleus
1670–80; < New Latin, masculine derivative of Latin solea sandal; see sole2
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
muscle of the calf of the leg, 1670s, Modern Latin, from Latin solea "sole" (see sole (n.1)). So called for its flatness.
Online Etymology Dictionary, © 2010 Douglas Harper