Unloquacious

Don't use a big word when a singularly unloquacious and diminutive linguistic expression will satisfactorily accomplish the contemporary necessity.

Source: tatoeba (5275580)

But to-night the elderly servitor was solemn and unloquacious beyond his custom.

Source: tatoeba (12128833)

Between two such unloquacious persons, dialogue was naturally slow at first, but they had a long drive before them.

Source: wiktionary

The Arab odd-job boy finished his watering and silently returned to the house. Like that young Hardcastle, Freddy thought. Like Hardcastle, the gardener’s boy of Freddy’s youth, who had moved back and forth, remotely attending to things, unloquacious, unsmiling, totally unwilling to conspire in Freddy’s games.

Source: wiktionary