You don't give a context which might be important because in English history the "illegitimate pretender" could be referring to someone who is making an illegal claim to the throne like Oliver Cromwell who tried to seize power.So without context we cannot know the exact meaning.

P.S. Whenever a revolution has deposed a monarch, as in the French Revolution and the Russian Revolution, large numbers of former nobility have to flee the country and live in exile, often losing most of their family fortunes. When this happens, you have large numbers of exiled aristocrats who act like aristocrats but are more or less out of money. This in turns makes it possible for ordinary con artists to pretend that they, too, are exiled, penniless aristocrats and swindle people. Such a pair of con artists--the Duke and the Dauphin--are characters in Mark Twain's famous book, "The Adventures of Huckleberry Finn." In this case, they are phonies and frauds--however, they are posing as legitimate heirs.

Sunday, May 31, 2015.... Heba, you have posted an interesting question regarding the illegitimate pretender? Actually, I find the collective words as an oxymoron. An oxymoron is a situation that can’t exist simultaneously. An oxymoron is simply a figure of speech that can be assumed to be contradictory. For this reason the proximity of the words illegitimate and pretender is considered as an oxymoron due to the fact that both words are in the same colloquial family. Extended explanation: illegitimate and pretender are synonyms for each other with a presumed understanding that each word could stand alone with a definition of being unauthorized or unacceptable. I would love the opportunity to have classes with you to further discuss the method of building a strong, dynamic, and solid vocabulary in conversational English. You see, I have a vociferous desire to improve my vocabulary while helping English language learners use useful vocabulary in a specific manner. Thanks for the opportunity to respond to your post. Dr. Priscilla

Please give us the context--where did you see it, and what was the full sentence in which it occurred? I've usually seen it in the (old-fashioned) context of royalty (kings) and royal succession. In this context, "pretender" doesn't have it's usually meaning. ahdictionary defines it as "One who sets forth a claim, especially a claimant to a throne." Such a claim could be valid or not. Every king would like to have a son lined up, ready, and waiting to take over the throne. Historically, that isn't always the case. If there is no obvious heir to the throne, it becomes a legal battle governed by the laws of royal succession in that country. There could be several people who are claimants ("pretenders.") In this context, "legitimate" does NOT mean "valid." It has the specific meaning of an illegitimate child--that is to say, the biological child by someone who was not the king's wife. Apparently--I'm no expert on this!--in some cases, if a king have any legitimate children, there might be situations under which an illegitimate child could legally be entitled to inherit the throne. And it has actually happened: http://history.stackexchange.com/questions/12521/bastard-children-who-became-kings The word "bastard" in modern English almost always means "nasty, mean person," but it literally means "illegitimate child," so if you explore this topic you will see the word "bastard" used.

It's a child whose parents were not married to each other at the time of his/her birth.