rattlesnake

English rattlesnake is simply rattle plus snake, joined to describe a snake with a rattle. Snake itself traces back through Middle English snake and Old English snaca to Proto-West Germanic *snakō, from Proto-Germanic *snakaną.

The word hasn't shifted much in meaning — it was formed by combining two existing English words, rattle and snake, to plainly name the creature by its most distinctive feature.

It shares its second half with every other English word for the reptile called snake, connecting it to that same Old English snaca lineage.