E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A Roman emperor; so called because he wore a military sandal called a calga, which had no upper leather, and was used only by the common soldiers. (12, 3741.)
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 levioribus, quibus adolescentior non fuerat in exercitu Germanici patris sui. And the calig were also proper to the monastic bodies; for we read in the ancient Glossarium, upon the rule of St. Benedict that calig were tied with latchets.Scott: Waverley, xlviii.