クソ真面目 まじめではありませんでした
kusomajimedewārimasendeshita

Desu Copula
na-Adjective

The polite, past negative form of クソ真面目

1. Was not earnestly serious.; Was not overly conscientious.; Was not a goody-two-shoes.

Past/Perfective
Negative
Polite
Formal Register