My "Bill Posters" phrase was an attempt to link back to phrases, but to go to one more directly, a phrase that has always fascinated me is "he knows a hawk from a handsaw". Seems that handsaw here is a corruption of Hernshaw, which is a heron. Fair enough, to know the difference between a hawk and a heron would hardly be difficult so the phrase is presumably used with ironic intent. But whence came it in the first instance I wonder?

dxb.