A Roman emperor; so called because he wore a military sandal called a caliga, which had no upper leather, and was used only by the common soldiers. (12, 37-41.)
“ `The word caligæ, however,' continued the Baron ... `means, in its primitive sense, sandals; and Caius Cæsar ... received the cognomen of Caligula, a caligis, sive caligis levio'-ribus, quibus adolescentior non fuerat in exercitu Germanici patris sui. And the caligoe were also proper to the monastic bodies; for we read in the ancient Glossarium, upon the rule of St. Benedict ... that caligoe were tied with latchets.” —Scott: Waverley. xlviii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Caligula from Infoplease: