guest (n.)
Old English gæst, giest (Anglian gest) "guest; enemy; stranger," the common notion being "stranger," from Proto-Germanic *gastiz (cognates: Old Frisian jest, Dutch gast, German Gast, Gothic gasts "guest," originally "stranger"), from PIE root *ghos-ti- "stranger, guest; host" (cognates: Latin hostis "enemy," hospes "host" -- from *hosti-potis "host, guest," originally "lord of strangers" -- Greek xenos "guest, host, stranger;" Old Church Slavonic gosti "guest, friend," gospodi "lord, master"); the root sense, according to Watkins, probably is "someone with whom one has reciprocal duties of hospitality," representing "a mutual exchange relationship highly important to ancient Indo-European society." But as strangers are potential enemies as well as guests, the word has a forked path.

Spelling evolution influenced by Old Norse cognate gestr (the usual sound changes from the Old English word would have yielded Modern English *yest). Phrase be my guest in the sense of "go right ahead" first recorded 1955.