Meaning

noun a style of orthography characterized by somewhat rounded capital letters; found especially in Greek and Latin manuscripts of the 4th to 8th centuries

adjective relating to or written in majuscule letters (which resemble modern capitals)

originAttested 1712, from Late Latin unciales (“uncials”), unciales litterae (“uncial letters”) (Jerome), plural of uncialis (“pertaining to one twelfth part, ounce, or inch”), from uncia (“one twelfth part, ounce, inch”). The literal meaning is unclear: some references indicate "inch-high letters", but see “Uncial script” in Wikipedia.

Etymology from Wiktionary, CC BY-SA 3.0