What is exact meaning of "ad hoc" in programming?
"Ad hoc" is a Latin phrase which can apply to anything, not just programming specifically. It basically means, more or less, something that was made up on the fly just to deal with a particular situation, as opposed to some systematic approach to solving problems.
Regarding programming specifically, it's probably similar to what Joel Spolsky recently called "Duct tape Programming"
It basically means writing some quick and dirty code without the intention of reuse. User-entered queries are usually the main example. Another common occurrence is a utility to convert data sets from 1 form to another, which will have no use when the conversion is done.
Formed temporarily for a specific, non-continuing purpose, as an ad hoc committee on ice removal.
Impromptu, not planned, improvised, as an ad hoc attempt to remove the ice with a screw-driver.