Origin and history of wo

1620s, a cry to call attention from a distance, a variant of who. Obsolete in the original sense. As a command to stop a horse, it is attested from 1843, a variant of ho. As an expression of delight or surprise (1980s) it gradually superseded wow, which was popular 1960s.

mid-13c., a variant of wei (late 12c.) "misery, trouble, grief, wretchedness," from the interjection wei! "ah! oh! alas!", Old English wa!, Northumbrian wæ, representing a common exclamation of lament in many languages. Compare Latin væ, Greek oa, German weh, Lettish wai, Old Irish fe, Welsh gwae, Armenian vay. Old French ouai, Italian and Spanish guai are considered to be from Germanic.

It was used in denunciations (woe to the vanquished) and thus interjectionally. Also compare wellaway. By late 14c. as "a heavy calamity, an affliction." Related: Woes. Paired alliteratively in weal and woe "prosperity or adversity" from mid-13c.