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Why is 口が軽い the opposite of 口が堅い ? It seems interesting to me why 軽い/堅い would be chosen for this phrase.

As far as I know, most of the time phrases that are coupled with 軽い usually have the opposite meaning paired with 重い. I know one case where it is paired with 激しい as in 軽く/激しく運動する.

What would be the reason as to why 口が重い might not be the appropriate choice to use for the opposite phrase of 口が軽い?

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I think we also have 口が重い (but this might not be the opposite of 口が軽い・・・) – user1016 Jul 24 '12 at 15:42
@Chocolate: That might be a reason then. If 口が重い is already taken and fits a different meaning better.. – Chris Harris Jul 24 '12 at 15:49
There are actually antonym phrases with 軽い, 重い: 腰が軽い, 腰が重い, 尻が軽い, 尻が重い. However, 尻が軽い also has a more specific sense. And there is also another pair with 軽い, 堅い: 身持ちが軽い, 見持ちが堅い. – user458 Jul 24 '12 at 16:56
@sawa: Do you know a reason why 緩い isn't involved in these comparisons? That seems like a viable opposite of 堅い. – istrasci Apr 9 '13 at 17:41

Well actually, 口が重い is the opposite of 口が軽い if the meaning is to be talkative. From here (see definition 2)

口が軽い(くちがかるい) 1.言ってはいけないことまで、軽率に喋(しゃべ)りがちである。秘密などを軽々しく話してしまいがちである。また、そういう性格である。 反:■口が堅い 2.良く喋る。多弁である。 反:■口が重い

The idea here is that 口が軽い can mean two things, while 口が堅い and 口が重い only have one meaning respectively. I am assuming that this is because a talkative person can imply that they say "too much" (i.e. says things they shouldn't say). However, a person who can keep a secret, may not necessarily be untalkative. Therefore, it is natural for 口が軽い to have two meanings, while 口が堅い and 口が重い cannot do this.

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Hmm... we don't say 口が軽い to mean someone's talkative/chatty/all mouth, in modern standard Japanese at least... we say 口数[くちかず]が多い or おしゃべりだ instead. We use 口が軽い to refer to someone who has a loose tongue/a big mouth/can't keep secrets (in modern standard Japanese at least). おしゃべりだ can be used to say both "口数が多い" and "口が軽い". So... what I want to say here is, if you're in Japan or talking to Japanese in some other places, I think you should avoid using the phrase like 口が軽いですね to mean "You're (s/he is) talkative", because the hearer would surely take it as "You have (S/he has) a loose tongue". – user1016 Jul 31 '12 at 5:23
@Chocolate: Good advice and I have to admit "口が軽い" sounds negative. I've always liked the phrase 口から生まれた though :). – Jesse Good Jul 31 '12 at 5:35

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