Translation:What are you doing?
Taiwanese use 干 a bit. But I was taught in Taiwan that it can be impolite. Taiwanese politeness is influenced by Japanese culture, so they are more sensitive to these things.
However, friends often use this to one another, particularly youth. For them, it may be more "informality" than "impolite" among peers. I would NEVER use it with a senior, teacher, businessman, etc. though.
So 干 can 干犯!
Nice explanation:) "幹" is impolite only if you just say out single word or with some fixed phrases which are originally swear words. It is very normal to use this word in "樹幹", "主幹" or any other daily words require this characters.
In my opinion many Taiwanese youths use "幹" to express exclamation or other strong moods, though we are taught that this is rude :)
Taiwanese here. "做什麼" and "幹什麼" are both common in Taiwan, and "幹什麼" is more colloquial, but not rude at all. The word is not pleasing only when you really intend to swear. Actually "幹" as a swear word (f**k) is not mandarin but Min-Nan dialect, the character is just a transcript.
By the way, please let me fix your misused characters: "什麼" and "甚麼" are both correct, but the former is used more frequently. "乾" means "dry," although it as well as "幹" is written as "干" in simplified Chinese. The last one is "是不是," which I think is just a typo.
Actually, if your drop the 在, the meaning will be different in Chinese's ears and tone is slightly different as well. "你在做什么?" is simply a question that asks "what are you doing now?" But if you say "你做什么?", in my ears, you are accusing this person. Mostly like when you say it in English "what the hell are you doing?!", such as when you see a thief stealing, you can say "你做什么?!"