You only have (half?) an hour after posting to edit. After that it's locked in, and only mods can edit.
I only briefly looked at the link, saw that the banjo in question is a spade named for it's banjo-like shape. It was a little something to add.
http://english.stackexchange.com/questions/3100/what-is-the-origin-of-couldnt-hit-a-cows-arse-with-a-banjo
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Edit >> go advanced
But I suppose it's too late now.