A cyclist who, despite biking (and often even bike commuting) in every imaginable weather condition, refuses to have a pair of mudguards installed on his/her bike. As a result, all of his/her pants, shirts, jackets, backpacks, etc., are marked by a long, brown streak down the center.
By extension: A cyclist who, for fear of making himself or his bike seem less pro, refuses to add any accessories to it - accessories that many others consider useful, such as carrier racks, baskets, kickstands, lights, etc.
By extension: A cyclist striving to give of a pretend air of professionalism.
In the practical bicycle culture of Amsterdam, Berlin, and many other European cities, the typical North American streak biker would be met with looks of bewilderment and derision.