To act as though one knows how to do s.t. when he doesn't; to try to teach s.o. s.t. when you don't have the right to teach or don't have adequate knowledge; to try to see who has the most know-how.
You are just acting as though you know how to fix a motorcycle, when in fact you don't know anything.
Don't try to teach me about sewing because I know how to sew better than you do.