It has been said that the title for the film comes from a combination of two other movies.
Tarantino, not very good at speaking French, always referred to Louis Malle's "Au revoir, les enfants" simply as "that Reservoir movie". He is also a fan of Sam Peckinpah's "Straw Dogs".
So he combined the two titles to get "Reservoir Dogs".
I heard a theory that a 'reservoir dog', ie an animal that lives in/near a reservoir, is a rat, and one of the main storylines in the film is trying to work out who the 'rat' is who grassed them up.
I saw an interview with tarrintino in wich he disscused
RESERVOIR DOGS he said that it did not mean anything
really, he would just let people draw there own conclusions.