Old English hlot "object (anything from dice to straw, but often a chip of wood with a name inscribed on it) used to determine someone's share," also "what falls to a person by lot," from Proto-Germanic *khlutom (cf. Old Norse hlutr "lot, share," Old Frisian hlot "lot," Old Saxon hlot, Middle Dutch, Dutch lot, Old High German hluz "share of land," German Los; Old English hleotan "to cast lots, to foretell"), of unknown origin. The object was placed with others in a receptacle, which was shaken, the winner being the one that fell out first. Hence, to cast lots. In some cases the lots were drawn by hand. The word was adopted from Germanic into the Romanic languages (cf. lottery, lotto). Meaning "choice resulting from the casting of lots" first attested c.1200.
Sense of "plot of land" is first recorded 1630s (distribution of the best property in new settlements often determined by casting lots), that of "group, collection" is 1725, from notion of auction lots. The generalized sense of "great many" is first attested in 1812. To cast (one's) lot with another is to agree to share winnings.