Tonight we are going to hear a guitarist. It is a special occasion and therefore I will wear a suit (traje). The suit coat is called a saco. It can also be a sports coat.
Saco has some meanings outside of a fancy piece of clothing which does not seem to fit with a suit coat. If you were to pillage and plunder it is the verb saquear. Saco is I sack (or plunder).
Saco is most commonly thought of and used for a bag or sack.
I will either wear a suit coat or a bag to the event tonight. Either way it will be a saco.