Although this term does come into English from French, the deeper (but still secure) etymology is Scandinavian, "flana" - to wander.
The term in French and in 19th Century English was ambiguously loaded. It could be positive, as in a person of leisure, or negative, as in an idler or a superficial person.
What makes this term interesting to me is that it has acquired an additional level of meaning which can be read as a play on "wandering/wondering". A variety of writers have explored the term. Feminist critical theory has used the concept to question issues of gender and power in urban spaces in a kind of crossover with psychogeography. The attitude of a "flâneur" is evoked in philosophy to describe an attitude of investigation detached from ideology.