"person who receives guests," especially for pay, late 13c., from Old French oste, hoste "guest, host, hostess, landlord" (12c., Modern French hôte), from Latin hospitem (nominative hospes) "guest, stranger, sojourner, visitor (hence also 'foreigner')," also "host; one bound by ties of hospitality."
This appears to be from PIE *ghos-pot-, a compound meaning "guest-master" (compare Old Church Slavonic gospodi "lord, master," literally "lord of strangers"), from the roots *ghos-ti- "stranger, guest, host" and *poti- "powerful; lord." The etymological notion is of someone "with whom one has reciprocal duties of hospitality" [Watkins]. The biological sense of "animal or plant having a parasite" is from 1857.
"a multitude," especially an army organized for war, mid-13c., from Old French ost, host "army" (10c.), from Medieval Latin hostis, in earlier use "a stranger, foreigner," in classical use "an enemy," from PIE root *ghos-ti- "stranger, guest, host."
It replaced Old English here (see harry (v.)), and in turn has been largely superseded by army. The generalized meaning of "large number" is first attested 1610s. The Latin h- was lost in Old French, then restored in Old French and Middle English spelling, and in modern English also in pronunciation. Lord of Hosts translates Hebrew Jehovah Ts'baoth (which appears more than 260 times throughout the Bible) and seems to refer to both heavenly (angelic) and earthly hosts.
"body of Christ, consecrated bread," c. 1300, from Latin hostia "sacrifice," also "the animal sacrificed, victim," probably ultimately related to host (n.1) in its root sense of "stranger, enemy." Applied in Church Latin to Christ, in Medieval Latin to the consecrated bread.
"to serve as a host," early 15c., originally in the sense "give entertainment, receive as a guest," from host (n.1). Related: Hosted; hosting.