Why do people blow on their fingernails and then brush it on their coat collar?

When done unconsciously, it's to blow little fuzz bits or crumbs off your fingertips, and then buff them on your clothes. As a deliberate gesture, it is used to show a feeling of superiority and expertise,usually as a little jokingly show-off thing, but done aimed at someone you dislike or consider inferior , it's sort of a swaggering put-down.It has it's origins as a class-concious gesture, when only the upper classes could afford manicures , then was taken up by the working class as a passive- aggressive way to mock their " betters " .