幹嗎

幹 (“to do”) + 嗎 (“(dialectal Mandarin, e.g. Tianjin) what”). Colloquial; the formal equivalent in Standard Mandarin is 做什麼 or 幹什麼.

The second element is cognate with 麼 (me) in Standard Mandarin 什麼／什么 (shénme, “what”), and is usually written as 嘛 when writing Tianjin dialect. Dictionaries only consider toneless ma (declarative particle) as the only proper use of 嘛. All other senses, including the dialectally influenced pronunciation of má ("what") and the interrogative particle of ma, are considered incorrect. However, there is much confusion in actual writing, possibly with 幹嘛 being used more commonly for this sense.

幹嗎

1. (colloquial) what is (someone) doing; what is going on

   你現在幹嗎呢？ [MSC, trad.]
   你现在干吗呢？ [MSC, simp.]

   Nǐ xiànzài gànmá ne?

   What are you doing now?

2. (colloquial) what is wrong; what is up

   你幹嗎呀？／你干吗呀？  ―  Nǐ gànmá ya?  ―  What's up with you?

幹嗎

1. (colloquial) why; why on earth; whatever for

   你幹嗎折磨自己啊！ [MSC, trad.]
   你干吗折磨自己啊！ [MSC, simp.]

   Nǐ gànmá zhémó zìjǐ a!

   Why are you torturing yourself?

幹嗎

1. 幹 (“to do”) + 嗎 (“question particle”)

   這事你幹嗎？／这事你干吗？  ―  Zhè shì nǐ gàn ma?  ―  Will you do this?

2. (vulgar) 幹 (“to fuck”) + 嗎 (“question particle”)
3. Alternative form of 幹嘛 (“to do + declarative particle”)