monster customer; extremely demanding customer
A customer who makes unreasonable demands, harasses staff, or behaves abusively. Often used in business and service-industry contexts.
Recently, more stores are struggling to deal with monster customers.
She was subjected to unreasonable complaints by a monster customer for hours.
Variant with interpunct; less common in running text.
Wasei-eigo (Japanese-made English) combining 'monster' and 'customer'. The exact origin is uncertain, but it became widely used in the 2000s to describe abusive customers.