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As a greeting first attested in 1859, attested first as a cry of sailors and huntsmen (first attested in the 1400s; compare e.g. huzzah, giddyup). Originally from Middle English yo, io, ȝo, yeo, yaw, variant forms of ya, ye (“yes, yea”), from Old English ġēa (“yes, yea”), from Proto-Germanic *ja (“yes, thus, so”); or perhaps from Old English ēow (“Wo!, Alas!”, interjection). Compare Danish, Swedish, German, Norwegian jo (“yes (flexible meaning)”), Dutch jow (“hi, hey”) and Dutch jo (“hi, hey”). More at yea, ow, ew. Modern popularity apparently dates from the early 20th century in Philadelphia, PA; amongst the Italian Community there, where io, with (as opposed to in standard Italian language) the stress on the ultima, was a common salutatory response among residents, particularly young males. This usage was apparently reinforced by the aforesaid English terms. It has been claimed to have been a common response at roll calls during World War 2 (see definition 4), and then most intensely attested in Philadelphia, Pennsylvania; it thence spread globally from American dominance of pop culture post-WWII.

From a non-rhotic pronunciation of your with the dough-door merger.

Ellipsis of such phrases as your friend/buddy, your brother/sister, your mom/dad, etc., referring to someone (whether present or absent, male or female) who is related to the interlocutor in some way. All such forms were then shortened to your (pronounced [joʊ], "yo", which see above). Eventually, the original meaning "your [someone]" was lost and broadened to "that person".

From Russian ё (jo).

From irregular romanization of the standard Mandarin pronunciation of Chinese 龠 (yuè).