A little history of the term: Manila envelopes are called such because they were originally made of a coarse brown paper called Manila paper. Manila paper, in turn, was made of Manila hemp, or what the locals call abaca. Abaca is a plant that is native to the Philippines, and its fibers are used as raw material for a wide variety of products, including the aforementioned paper, rope, and textile.

The Philippines used to export a lot of abaca based paper, thus people started calling it Manila paper. Manila being the capital city and main trading port of the Philippines.