This does not mean, as is commonly supposed, to evade a straight answer to a question. It means to form a conclusion by making an assumption which is as much in need of proof as the conclusion itself. Logicians call this petitio principii.
"Thus to say that parallel lines will never meet because they are parallel is simply to assume as a fact the very thing you profess to prove" (Brewer).
A single word can be used in a question-begging way: Reactionary, victimisation, aggression, imperialism and warmonger are common examples.