The word 'terrier' has a direct and earthy origin. It entered English in the early 15th century from the Middle French 'chien terrier', literally meaning 'earth dog'. The French 'terrier' itself derives from the Medieval Latin 'terrarius', an adjective meaning 'of the earth', which is built on the Latin noun 'terra' (earth, land). This etymology perfectly captures the dog's original function: a breed specialized in pursuing quarry such as foxes, badgers, and rats into their underground dens and burrows. The modern, capitalized use for a member of the Territorial Army is a 20th-century extension, humorously alluding to the idea of a defender of the home territory.

Structure: terr (earth) + ier (one who does/has) 'terr' is from Latin 'terra', meaning 'earth, land'. The suffix '-ier' is from Old French '-ier', an agent suffix meaning 'one concerned with or connected to'.