The term "filthy casual" is an internet slang used to describe someone who appears to be careless or uninformed about a particular topic, but actually knows quite a bit. It's often used in gaming communities when referring to players who appear to be newbies or not well-versed in the gameplay mechanics, but are secretly skilled and have been playing for years. The phrase is meant as an ironic jab at those who may seem uninformed on the surface, but actually possess a deep understanding of the subject matter.