In Chinese, which connotations does the made-up word "geeba" have?
Some Chinese friend told me that it may mean something bad, so I
wonder what it is.

Morning scrtchy-ga!
  You've asked a good question.  
We'd need additional information before we could venture an educated
comment for you. Chinese, as you may know, is a tonal language when
spoken, so the meaning of any word in Chinese depends on its tones
(your made up word has two syllables, which could be any combination
of at least four tones). In addition, there are many dialects spoken
in Chinese (Mandarin, Cantonese, etc) so it would depend on which
dialect this made up word is spoken in. One final complication is
there are several romanization systems used to transcribe the word
into an alphabet-type language. It can be difficult to know which
romanization system you have used. We might have better luck at
commenting on your question if you could submit an audio file (like a
.wav or .wma file) in which you or your Chiense friends say the word
several times.

hey scrtchy! I'm Chinese, from Singapore. Maybe I can help answer your question. 

I've been reading that phrase to myself over and over, and thought to
myself, "What phrase sounds like that and has a possible bad(or
vulgar) meaning?".

Then a phrase came into my head. This connotations is not actually
Chinese, but a Chinese dialect (Hokkien). The correct pronounciation
is actually "chee bye", which means something vulgar.

We use this phrase alot here in Singapore, and this phrase refers to a
women's private part. So when you hear a Chinese scolding someone else
with this phrase, he/she is actually swearing by a women's private
part.

I don't think the comment of kl1985 is correct. As a Chinese, I think
I can explain correctly to you. "geeba" is a vulgara word in Chinese.
It means the dick or penis in English, not the women's private part.
That is a dirty word, same as say "dick" to others. That's all. No
more meaning.

okay.erm ,first i think i can answer you .
i am from China ,the southern China
and it means "dick"...thats all

oh... so it isn't Hokkien dialect eh? my mistake.. heh :D

Here is one possibility.  Jiba is pinyin, the written form of Chinese
that uses our alphabet.  It also happens to sound exactly like
"geeba".  Ji and ba can both have many different meanings, given
different tones.  Ji can mean chicken and ba can be put at the end of
a sentence, changing the whole sentence to "I presume that..."  We
were taught early in our Chinese class to never say, "I presume you
like to eat your mothers chicken" or "Ni xihuan chi nide muqin de ji
ba".  It was explained to us that another word, jiba, probably with a
different tone, is the derogatory form of vagina.  So if a Chinese
person hears you say "geeba" they could think you are saying jiba,
which can mean "pussy".

yes,geeba is just Jiba---dick.
I am from northern China