Hovercraft

When a really, REALLY fat girl (or guy or whatever) wears a dress with frilly skirt bits that hang down so the overhang makes it difficult to see their legs and they look like the skirts on a hovercraft as they're walking around. You show your appreciation and recognition of this fashion by making subtle BRRRRRRRR hovercraft noises as they walk past.

Jesus, Dave, have you seen what the whale in finance is wearing today? Hovercraft-mode ENGAGED! BRRRrrrrrrrr.....