He found Cronshaw dressed, sitting in his hat and great-coat on the bed, with a small, shabby portmanteau, containing his clothes and books, already packed: it was on the floor by his feet, and he looked as if he were sitting in the waiting-room of a station.
— W. Somerset Maugham, Of Human Bondage, 1915
A portmanteau is a traveling bag with two compartments, one whose name derives from a Middle French word formed by combining the verb porter (“to carry”) with the noun manteau (“mantle”). It is usually made of leather.
You may be more likely to hear portmanteau used for a word that is formed by combining the parts of two other words, such as brunch for breakfast and lunch. We can thank the author Lewis Carroll for the extended use, which he created on the idea that a portmanteau has more than one word “packed” into it, much the same way things are packed together into a traveling bag.