What is the opposite of "outgoing"?

The answer to this question depends upon how it is being used. Generally speaking, it is an adjective. If you're talking about a person, then the opposite would be "reserved" and "not very socialable". If you use "extrovert", then the opposite would be "introvert". There can be many reasons for someone being "reserved". Example of how to use this word: I'm a very outgoing person because I like to go out to dinner with my friends frequently. But, my brother is reserved because he prefers to eat at home more often.

On the hand, "outgoing" can be used to mean the opposite of "incoming". This is seen often regarding regular ("snail") mail and financial records. "Incoming" here means things like refunds, pay checks, salaries, money received from land you own and let others use (called "rent) and so on. Another word for "incoming" that is sometimes used is" inflow". However, "outgoing" here means "bills" or "expenses", which can include electricity, telephone bill, car loans, student loans, attorney fees, food, clothing, and more. Some people even use the word "outflow" to mean the same as "outgoing" in this use.

To summarize, the word "outgoing" can have more than one meaning. It depends upon how the speaker or writer is using it.