Whatcha = what did you

This is a beautiful example of conversational English. This expression has three important parts to it: omission, linking, and a neutral vowel (the schwa).

With common expressions or combinations of words, sometimes you can omit unnecessary parts of it. Did is not an important verb to understand the sentence, so it was omitted. Questions often omit words in this way.

(Are) You coming?

(You are) Not sure?

Another important part of spoken English is the use of linking sounds. Linking sounds join two words together to sound like one. An example is seen with -t and y-. When one word ends with a t and the next word begins with a y, they combine to make ch.

What you = whachou

To complete this excellent example, it uses the most common English vowel, the schwa. When a vowel sound is not stressed in a word, the vowel sounds like /uh/. For example, police is not said poh-leece, but puh-leece.

What is the important part of "whachou;" you is not so important, so you don't stress the vowel in it. Hence whachou becomes "whachuh" or "watcha."

Note: Whaju is another common sound from this combination. -d + y- = j.

Whaju do? = What did you do?