A victim. The consecrated bread of the Eucharist is so called in the Latin Church because it is believed to be a real victim consisting of flesh, blood, and spirit, offered up in sacrifice. (Latin, hostia.) At the service known as the Benediction it is set up for adoration, and with it the blessing is given in a transparent vessel called a “monstrare.” (Latin, monstrare, to show.
An army. At the breaking up of the Roman Empire the first duty of every subject was to follow his lord into the field, and the proclamation was banire in hostem
(to order out against the foe), which soon came to signify “to order out for military service,” and hostem facere
came to mean “to perform military service.” Hostis (military service) next came to mean the army
that went against the foe, whence our word host.
Like the leaves of the forest, when summer is green, That host with their banners at sunset was seen; Like the leaves of the forest, when autumn has blown, That host on the morrow lay withered and strown.
Byron: Destruction of Sennacherib, stanza 2.
To reckon without your host.
To reckon from your own standpoint only. Guests who calculate what their expenses at an hotel will come to always leave out certain items which the landlord adds in.
Found in few minutes, to his cost, He did but count without his host.
Butler: Hudibras pt. i. canto iii. lines 22-3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894