A bounded body of work with an intended end. Also to throw forward — a project is something cast ahead of where you stand.
Latin projectum (a thing thrown forward), from proicere (to throw forward): pro- (forward) + iacere (to throw); a project was first the throw itself, before it became the bounded work aimed at where the throw lands.
A bounded body of work cast ahead of where the worker currently stands. The throwing is the honest part of the metaphor: a project is aimed at a place not yet reached, which is why it can fall short.