From Longman Dictionary of Contemporary Englishinform on/against somebody phrasal verb formalPMSCPto tell the police or an enemy information about someone that will harm them He denied that he had ever informed on his neighbours. → inform→ See Verb table
Examples from the Corpus
inform on/against • And they had to be kept informed on a regular basis about changes in the competitive picture and performance expectations.• He was well informed on civil and constitutional law.• His father was a quiet man, but he was surprisingly well informed on current issues.• Luke O'Malley had committed the unforgivable sin - he had informed on his friends.• It was the press that later seemed to always side with the blacks, informing on police who committed excesses.• The zeal to conform and inform on their neighbours gave the Inquisition many willing helpers.• He also was quickly released, but he informed on two other well-known villains, MacDonald and Collins.• How all occasions do inform against you, George! 