Ad Hoc, Ad Lib, Ad Nauseam: So You Think You Don't Speak Latin? by John Parker
Ad libitum (ad lib) At pleasure. In a stage play, a line or comment which does not appear in the script, but which the actor speaks at his own pleasure or that of the director. If you are offered something ad libitum, you are free to accept or reject as you see fit; written on a doctor's prescription, ad lib means 'to be taken freely'.