O.E. gæst, giest (Anglian gest) "guest, enemy," the common notion being "stranger," from P.Gmc. *gastiz (cf. O.Fris. jest, Du. gast, Ger. Gast, Goth. gasts), from PIE base *ghostis "strange" (cf. O.C.S. gosti "guest, friend"), also preserved in L. hostis "stranger, enemy," and hospes "host," from hosti-potis "host, guest," originally "lord of strangers." Spelling evolution infl. by O.N. gestr (the usual sound changes from the O.E. word would have yielded Mod.Eng. *yest). Phrase be my guest in the sense of "go right ahead" first recorded 1955.