late 13c., from Anglo-French rounde, Old French roont (12c., Modern French rond), probably originally *redond, from Vulgar Latin *retundus (source also of Provençal redon, Spanish redondo, Old Italian ritondo), from Latin rotundus "like a wheel, circular, round," related to rota "wheel" (see rotary).
As an adverb from c. 1300; as a preposition from c. 1600. In many uses it is a shortened form of around. The French word is the source of Middle Dutch ront (Dutch rond), Middle High German runt (German rund) and similar Germanic words.
Of numbers from mid-14c., from earlier sense "full, complete, brought to completion" (mid-14c., notion of symmetry extended to that of completeness). Round trip is by 1844, originally of railways. Round heels attested from 1926, in reference to incompetent boxers, 1927 in reference to loose women, in either case implying an inability to avoid ending up flat on one's back.