means To rise, expand, or bulge outward — and, as a noun, the rolling waves that travel across open water far from where they were born; also, as old-fashioned slang, something excellent or a fashionably dressed person.

from From Old English 'swellan,' to grow larger or rise up, a Germanic word with cousins in Old Norse 'svella' and German 'schwellen.' The sea sense — long, smooth waves carrying a distant storm's energy — comes straight from this idea of water heaving and rising. The slangy 'swell!' (meaning grand or first-rate) bloomed in the 19th century, drawn from 'swell' as a noun for a puffed-up, well-dressed gentleman who looked, quite literally, like he was swelling with importance.