クレーマー

noun

chronic complainer; person who makes unreasonable complaints

Wasei-eigo from English 'claimer'. Refers to someone who habitually complains, often unreasonably, especially to businesses or service providers. Carries a negative connotation.

wasei-eigo (Japanese-coined English), from claimer

• あの客きゃくはクレーマーだから、対たい応おうに気きをつけて。

  That customer is a chronic complainer, so be careful when dealing with them.

• クレーマーからの電でん話わが毎まい日にちのようにかかってくる。

  We get calls from unreasonable complainers almost every day.