also: ぼろくそ
na-adjective, noun
Strongly negative colloquial evaluation; often used with 言う to mean 'to badmouth' or 'to tear to pieces'. The literal components mean 'rags and shit', giving it a vulgar, emphatic tone.
あの映画はボロクソに言われてるね。
That movie is being trashed, isn't it?
上司に企画をボロクソにけなされた。
My boss tore my proposal to shreds.
na-adjective, noun
disparaging; demeaning; insulting
Describes speech or attitude that is harshly critical and belittling. Often used adverbially as ボロクソに to modify verbs like 言う, 書く, 批判する.
ネットでボロクソに書かれて落ち込んだ。
I got depressed after being slammed online.
Compound of ぼろ (boro, 'rags, tatters') and くそ (kuso, 'shit, excrement'). The combination intensifies the sense of worthlessness or contempt. The exact origin is uncertain, but it likely arose as a vulgar colloquialism.