"linear measure, one-twelfth of a foot," late Old English ynce, Middle English unche (current spelling c.1300), from Latin uncia "a twelfth part," from root of unus "one" (see one). An early borrowing from Latin, not found in any other Germanic language. Transferred and figurative sense of "a very small amount" is attested from mid-14c. For phrase give him an inch ... see ell.
"small Scottish island," early 15c., from Gaelic innis (genitive innse) "island, land by a river," from Celtic *inissi (cf. Old Irish inis, Welsh ynys, Breton enez).
"move little by little," 1590s, from inch (n.1). Related: Inched; inching.
A unit of length in the US Customary System equal to 1/12 of a foot (2.54 centimeters). See Table at measurement.