不客气 Búkèqi You're welcome

A: 中文的“不客气”是什么意思，一般在什么时候用呢？
    Zhōngwén de " bú kè qi " shì shénme yìsi, yìbān zài shénme shíhou yòng
    ne?
    What does "不客气" mean, and when it is used?
B: 当别人对你说“谢谢”时，你就要说“不客气”。意思就是你很愿意为别人做
    事，不用谢了。
    Dāng biérén duì nĭ shuō "xièxiè" shí, nĭ jiù yào shuō "bú kè qi." Yìsi jiù shì
    nĭ hĕn yuànyì wèi biérén zuòshì, bú yòng xiè le.
    You should say "不客气" when others say "谢谢" to you. It means that you
    are willing to help others.
A: 那刚才王俊没有说“谢谢”，为什么他老师说“不客气”？
    Nà gāngcái Wáng Jùn méiyŏu shuō "xiè xiè," wèi shénme tā lăoshī shuō
    "bú kè qi?"
    But Wang Jun didn't say "谢谢" just now and his teacher said "不客气" to him.
B: 哦？那王俊的老师怎么说的？
    Ò？Nà Wáng Jùn de lăoshī zĕnme shuō de?
    Really? What did his teacher say?
A: 如果你再敢迟到，我就对你不客气了！
    Rúguŏ nĭ zài găn chídào, wŏ jiù duì nĭ búkèqi le!
    If you are late again, I won't be so easy on you.

Analysis
The Chinese phrase "不客气" has two meanings and in this joke "A" doesn't distinguish the two meanings. The first meaning is you're welcome, which is used when others say thank you to you. The second meaning is unkind or impolite which is used when someone is angry and unkind or impolite.