http://www.etymonline.com/index.php?term=ad+hoc&allowed_in_frame=0 indicates "ad hoc" literally means "for this".
ad hoc
adj. (from Latin) arranged or happening when necessary and not planned in advance
More often "ad" translates as "to". However, prepositions can be very idiomatic and may not always translate consistently one-to-one.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.