expression, i-adjective
taciturn; reticent; slow to speak
Describes a person who is not talkative, often reluctant to speak or slow to open up. Can be used for a reserved personality or a temporary unwillingness to talk.
彼は口が重いので、会議ではあまり発言しない。
He is taciturn, so he doesn't speak much in meetings.
今日は何かあったのか、彼女は口が重かった。
She was unusually reticent today; I wonder if something happened.