Literally it means: O, my father! and it is used in some places in upper Egypt to indicate exclamation.

The ُEnglish equivalent is "O my God"

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Note added at 1 day2 hrs (2012-11-19 16:48:15 GMT)
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يا is an exclamation article either in classical or colloquial Arabic

بوي is derived from أبي (my father) that consists of the word أب and the possessive pronoun ــي
Anyway, out of pronouncing the word more easily and frequently, some people dropped the همزة )أ( and inserted an extra واو that is pronounced with stress on lengthening it

By the same token, they pronounce the phrase يا أمي as يَــمَّــاي or
يَا مَّــاي but this time they inserted an extra الف
It should be knowon that there are many variant ways to pronounce the phrase "My mother" and "My father" either in Upper Egypt, Lower Egypt, or any other Arab country and its different regions.

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Note added at 1 day2 hrs (2012-11-19 16:52:09 GMT)
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Also, there are many exclamation expression that differ depending on the situation
For example,

يا بوي is said in case of excitement and shock and sometimes to express that you are fed up with something

يا عيني is said to express sympathy

and many other daily expressions