*ghos-ti-

archaic name for the Black Sea, from Latin Pontus Euxinus, from Greek Pontos Euxenios, literally "the hospitable sea," a euphemism for Pontos Axeinos, "the inhospitable sea." From eu- "good, well" (see eu-) + xenos "host; guest; stranger" (from PIE root *ghos-ti- "stranger, guest, host").

According to Room, The Old Persian name for the sea was akhshaena, literally "dark," probably in reference to the sudden, dangerous storms that make the sea perilous to sailors and darken its face (or perhaps in reference to the color of the water, from the sea being deep and relatively lifeless), and the Greeks took this untranslated as Pontos Axeinos, which was interpeted as the similar-sounding Greek word axenos "inhospitable." Thus the modern English name could reflect the Old Persian one.

Old English gæst, giest (Anglian gest) "an accidental guest, a chance comer, a stranger," from Proto-Germanic *gastiz (source also of Old Frisian jest, Dutch gast, German Gast, Gothic gasts "guest," originally "stranger"), from PIE root *ghos-ti- "stranger, guest, host" (source also of Latin hostis, in earlier use "a stranger," in classical use "an enemy"); the root sense, according to Watkins, probably is "someone with whom one has reciprocal duties of hospitality."

Spelling evolution influenced by Old Norse cognate gestr (the usual sound changes from the Old English word would have yielded Modern English *yest). Meaning "person entertained for pay" (at an inn, etc.) is from late 13c. Old English also had cuma "stranger, guest," literally "a comer." Phrase be my guest in the sense of "go right ahead" first recorded 1955.