Informer

wiktionary

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inform +‎  -er

etymonline

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informer (n.)

late 14c., enfourmer "instructor, one who teaches or gives advice," from inform (Middle English enfourmen) and also from Old French enformeor. Meaning "one who communicates information" is mid-15c.; sense of "one who gives information against another" (especially in reference to law-breaking) is c. 1500.