noun
monster customer; extremely demanding customer
A customer who makes unreasonable demands, harasses staff, or behaves abusively. Often used in business and service-industry contexts.
最近、モンスターカスタマーへの対応に苦労している店が増えている。
Recently, more stores are struggling to deal with monster customers.
彼女はモンスターカスタマーに理不尽なクレームを何時間も浴びせられた。
She was subjected to unreasonable complaints by a monster customer for hours.