木村：先生、そのスーツお似合いですね。いつもきれいな先生が、いっそうきれいに見えますよ。
先生：木村さん、心にもないお世辞を言わないで下さいよ。
Kokoro ni mo nai
Kimura: Sensei, sono suutsu o-niai desu ne. Itsumo kirei na sensei ga, issoo kirei ni miemasu yo.
Sensei: Kimura-san, kokoro ni mo nai o-seji wo iwanai de kudasai yo.
Kimura: That suit really looks good on you. You always look nice, but that makes you look particularly attractive.
Teacher: I wish you wouldn't say that kind of tiling, Kimura-san. I know you don't mean it.
The expression kokoro ni mo nai is used to describe the way in which someone says or does something that they don't really mean.
As a woman, ifs always nice to be flattered, but this feeling of satisfaction is by no means restricted to women alone. Men, children and even pets (when they are praised) enjoy it too. Sometimes though, there can be ulterior motives to flattery as, for example, when it is used as a means of persuading someone to do something for you.
Kimura-san is very good at complimenting people. When one of the office staff serves him some tea, for example, he'll casually comment "you make a lovely cup of tea, Miss O." Miss O will reply with something like, sonna kokoro ni mo nai koto wo itte (Honestly, you know you don't mean that), but if the truth were known I'm sure she's secretly happy about it And the next time she makes the tea shell try extra hard to make it well.
There are some people who say, watashi wa kokoro ni mo nai o-seji wa iwanai koto ni shite iru (I make a point of never saying anything I don't mean). They're always moaning about one thing or another: 'That woman's got no taste!" "That man is so slow at his work!" "The food they serve at that restaurant is terrible!" and so on. Just spending time with such people makes me begin to feel miserable. On the other hand, when I'm with someone who is a good flatterer, who says things like, sensei no koto sonkei shite imasu. Hontoo ni bunshoo ga o-joozu desu ne. (I really admire you. You're such a talented writer), although I'm aware that kokoro ni mo nai koto wo itte iru no (He's just saying it), I can't help feeling that he is perhaps at least partly serious.
I think maybe the secret to successful flattery is managing to avoid letting people know that kokoro ni mo nai o-seji (your compliments are only surface-deep).