na-adjective, noun which may take the genitive case particle 'no', noun
taciturn; reticent; quiet; untalkative
Describes a person who naturally speaks very little; not necessarily shy or unfriendly, just not talkative. Often used as 無口な人.
He is quiet, but a very kind person.
無口な性格だから、会話が続かないことが多い。
Because I have a taciturn personality, conversations often don't last long.
He's quiet, but he thinks very fast.
After the accident, she suddenly became withdrawn and quiet.
He has a taciturn personality and rarely talks to people he's just met.
無口 describes someone who is quiet or reticent, the opposite of having a strong 弁 (eloquence).