A portmanteau is a linguistic blend of words, where parts of multiple words or their sounds are combined into a new word. The term was first used in this linguistic sense by Lewis Carroll in “Through the Looking-Glass,” where Humpty Dumpty explains the concept of blending words to Alice. For example, “brunch” (breakfast + lunch) and “smog” (smoke + fog) are other examples of portmanteaux.

The exact first use of the term “laundromat” is not well-documented, but it is known that the concept of self-service laundry facilities became popular in the 1940s in the United States. The first laundromat is often credited to the Westinghouse Electric Corporation, which introduced the concept and promoted it widely.

“Laundry mat” is incorrect because it misunderstands the origin of the word. While “laundry” is correctly used, “mat” does not derive from “automat.” The term “laundromat” specifically implies a self-service laundry facility, and breaking it into “laundry mat” loses this connotation and the blend of “automatic” service, which was the novel concept when laundromats were first introduced.