1640s, "pertaining to an ounce," from Latin uncialis "of an inch, of an ounce," from uncia "a twelfth part" (see inch, n.1). In reference to letters, it is attested from 1712, from Late Latin litterae unciales (Jerome), probably meaning "letters an inch high," from Latin uncialis "of an inch, inch-high." As a noun, "an uncial letter," from 1775.