i-adjective
slow of speech; taciturn; reticent
Describes a person who is not talkative, speaks slowly, or is reluctant to speak. Often implies a reserved or cautious nature rather than a speech impediment.
彼は口重い人で、会議ではほとんど発言しない。
He is a taciturn person and hardly speaks at meetings.
初対面の相手にはどうしても口重くなってしまう
I inevitably become slow of speech with someone I'm meeting for the first time.
Compound of 口 (mouth) and 重い (heavy), literally 'heavy-mouthed'. The exact historical derivation is uncertain, but the meaning follows naturally from the components.