It isn't French, it's Latin, and it means 'formed or done for a particular purpose only'. A literal translation is 'to this'.

Directly translated from its Latin origin, ad hoc means "for this"—the term is generally used to imply that something is intended for a particular situation or purpose. Hence, an ad hoc committee is one that is formed to handle a particular issue. While this is a good ad hoc definition in general, ad hoc takes on a more precise meaning in different applications. In truth, the ad hoc definition is exactly that: ad hoc. Actually, ad hoc has several specific applications in the world of Information Technology: ad hoc networks, ad hoc reporting, and ad hoc testing. When it comes to computer networks, ad hoc is typically associated with wireless devices. When a connection is established between two or more computers without having a base station, they have formed an ad hoc network. The ad hoc definition still holds in this case because the connection is not permanent but rather has been formed temporarily to meet a particular need. Ad hoc reporting, also called ad hoc querying, refers to a system that permits users to customize a data query from a database instead of limiting them to preset, "canned" options. Since the results of the query are dependent on the specific query submitted, they are created for the specific moment, ad hoc