expression, i-adjective
rumormongering; gossipy
Describes a person who spreads rumors or talks about others behind their backs. Often used critically.
近所の人は口がうるさくて、すぐ噂を広める。
The neighbors are gossipy and spread rumors right away.
彼女は口がうるさいから、秘密は話さないほうがいい。
She's a gossip, so it's better not to tell her secrets.
expression, i-adjective
Refers to someone who constantly criticizes or complains about small things. Often used for a person who is hard to please.
上司は口がうるさくて、いつも細かいミスを指摘する。
My boss is nagging and always points out small mistakes.
あの先生は口がうるさいから、宿題を忘れると怒られる。
That teacher is faultfinding, so if you forget your homework you'll get scolded.
うるさい alone means 'noisy' or 'annoying'. 口がうるさい specifically targets the mouth as the source of annoyance, emphasizing talking too much in a gossipy or critical way.
Similar meaning of being outspoken or critical, but 口やかましい can sound slightly more formal or literary. 口がうるさい is more colloquial.
Compound of 口 (mouth) + が (subject marker) + うるさい (noisy, annoying). Literally 'the mouth is noisy', idiomatically meaning a person who talks too much in a bothersome way.