the word 'zealous' entered english in the early 16th century, derived from the medieval latin 'zelosus'. this, in turn, originates from the ecclesiastical latin 'zelus', meaning 'fervor, emulation'. the latin term was a borrowing from greek 'zēlos', which carried the dual meanings of 'ardor, zeal' and 'jealous rivalry'. the underlying semantic core is one of intense, passionate energy. the suffix '-ous', of latin origin, attaches to nouns to form adjectives meaning 'full of' or 'characterized by'. thus, the literal construction 'full of zeal' evolved from the concrete notion of competitive emulation to describe a modern abstract quality of fervent devotion and enthusiastic pursuit.

Structure: zeal + ous zeal (from latin 'zelus', meaning 'ardor, jealousy', from greek 'zēlos', meaning 'ardor, rivalry'); ous (adjective-forming suffix, from latin '-osus', meaning 'full of').