na-adjective, noun
slow of speech; reticent; taciturn
Describes a person who is not talkative, speaks slowly, or is hesitant to speak. The gloss 'prudent' likely refers to a cautious, reserved manner rather than wisdom. Often used with 人 or 性格.
彼は口重な人で、あまり話さない。
He is a taciturn person and doesn't talk much.
口重な性格だから、初対面の人とはなかなか打ち解けられない。
Because of my reticent nature, I find it hard to open up to people I've just met.