"fadat" comes from the word fada قدا which means to sacrifice or give up life for someone or to reach a goal. "fadat" فدات is short for "fadat besham" فدات بشم which can be used as a greeting but it is feminine and can be cause for concern if used by two men.
Generally it is not polite to use this word or phrase, and it is considered a low class type of greeting. Although it can be used appropriately in certain circumstances. For example a grandmother when greeting her grandchild might say: "ghorboonet beram, fadat besham, azizam" قربونت بشم فدات بشم عزیزم This shows her extreme affection for the grandchild.
In certain circumstances it carries a sexual connotation and this is where the impoliteness of the expression stems from. For example a man trying to pick up a woman may say: "fadat besham gigar" فدات بشم جیگر which is roughly equivalent to whistling for the woman. Of course most women will be offended by whistling or by this expression in such a circumstance.
It maybe best not to use this expression in fear of offending someone. If you are going to use it then carefully note how you have seen it being used and the reactions it evokes before using it.
On the other hand you being a Russian and a female, will not draw the same critical review using this expression as long as it is done in friendly circles. In fact it may make you extremely popular.