The word "swell" can have several meanings depending on the context. Generally, it refers to the action of increasing in size, volume, or intensity. For example, when a balloon is filled with air, it swells. It can also mean to become more impressive or to enhance. Additionally, in a more colloquial sense, "swell" can describe something as excellent or wonderful.
The word "swell" comes from the Old English "swealwian," which means "to swell," "to rise," or "to grow larger." It is related to various Germanic languages and has been used in English since the medieval period.