Why are they called manila envelopes?

Manila envelopes get their name from the manila fiber harvested from the abacá plant, which is a relative of the banana. The tough fibers of manila hemp make the envelopes and folders highly durable.

The abacá plant is native to the Philippines. This explains the origin of the word "manila," which comes from the city Manila, the capital of the country. Originally used for rope, the fibers of the abacá plant also make strong fabrics and fishing nets. All manila folders and envelopes are not made of manila fiber anymore. The term now refers to any cheap, unbleached beige folder or envelope.