Tarps, autos, bikes, photos, prams...

Helen is quite right to connect these shortened forms with the reduction of brassiere to bra. There is a linguistic term for this, which I cannot recall.

Part of what is not cool about these reductions is that "auto" just means "self" and "photo" just means "light" thus they cause confusion to a person trying to figure out what an automaton is (has nothing to do with cars) or what photosensitivity is (has nothing to do with pictures).