mermaid

From Middle English mermayde, literally "maid of the sea," built from mere ("sea, lake") plus maid.

The word has stayed remarkably literal since Middle English — it names exactly what it describes, a sea-maid, without much drift in meaning.

It's cognate with Dutch meermeid and Middle High German mermaget/mermeit (ancestors of German Meermagd and Meermädchen), and echoes the Old English meremenn, which already meant "mermaid, siren."