"fabled marine or amphibian creature having the upper body in the form of a woman and the lower in the form of a fish, with human attributes," "usually working harm, with or without malignant intent, to mortals with whom she might be thrown into relation" [Century Dictionary]; mid-14c., meremayde, literally "maid of the sea," from Middle English mere "sea, lake" (see mere (n.1)) + maid.
Old English had equivalent merewif "water-witch" (see wife), meremenn "mermaid, siren" (compare Middle Dutch meer-minne, Old High German meri-min), which became Middle English mere-min (c. 1200) and was shortened to mere "siren, mermaid" (early 13c.); the later mermaid might be a re-expansion of this. Tail-less in northern Europe; the fishy form is a medieval influence from the classical siren, and mermaids sometimes were said to lure sailors to destruction with song.
A favorite sign of taverns and inns at least since early 15c. (in reference to the inn on Bread Street, Cheapside, London). Mermaid pie (1660s) was "a sucking pig baked whole in a crust." Mermaid's purse for "egg-case of a skate, ray, or shark" is by 1825, perhaps originally Scottish, as it is first attested in Jamieson.