Isopropyl chloride should have been called secondary propyl chloride, right? The first iso must be isobutyl chloride, methinks.
In this case sec-propylchloride and isopropylchloride coincide. The prefix “iso” has several interpretations, but generally means that a terminal carbon (methyl group) is moved down the chain by one, which for propylchloride means that a methyl group is attached to what was carbon 1 (the one with the chloride).
So you could call it either sec-propylchloride or isopropyl chloride, neither would be wrong, but it is desired to standardise names when possible, and isopropyl was chosen. AFAIK the choice of iso- over sec- was arbitrary, but as Neretin says, trivial names can’t be wrong by definition. I suspect, although I don’t know, that iso was chosen because it was known that the molecule was similar to propylchloride or had the same formula (thus an isomer) before the actual structure was known.
One should be aware that “iso” is sometimes also used in a looser sense to refer to any member of the class of isomers of a compound.