Something that makes something else cold or cool, such as a refrigerating machine, or something that causes fear or suspense (like a scary movie or story).
From English 'chill' (to make cold, from Old English origins) plus '-er' (agent suffix indicating something that performs an action). The 'scary' meaning developed metaphorically in the 20th century.
The word 'chiller' shifted meanings in modern English—it started as a machine for cooling things, but now it's just as likely to mean a spine-tingling horror movie! Language meanings slide around like that all the time.
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