1520s, "slow," from Old French tardif "slow, late" (12c.), also the name of the snail character in the Roman de Renart, from Vulgar Latin *tardivus, from Latin tardus "slow, sluggish; late; dull, stupid," of unknown origin. Meaning "late" in English is from 1660s.
This word, not much used in English prose, is constantly employed in the U.S. and in Canada with reference to lateness in school-attendance. [Thornton, "American Glossary," 1912]
Related: Tardily; tardiness. Earlier form of the word in English was tardif, tardyve (late 15c.). Tardity "slowness of movement or action" is recorded in English from early 15c., from Old French tardete, from Latin tarditas.
It is the hypothetical source of/evidence for its existence is provided by: Latin gradus "a step, a pace, gait," figuratively "a step toward something, a degree of something rising by stages;" gradi "to walk, step, go;" Lithuanian gridiju, gridyti "to go, wander;" Old Church Slavonic gredo "to come;" Old Irish in-greinn "he pursues."