Thanks Don, Now it makes more sense. I suppose that singers passing on that popular phrase in the ballads would not necessarily need to have known the details. The weapons all seem to be "thrust" into people after the business with the straw, but it seems to me that the idea might have become a formula to be slotted in anywhere a sharp weapon is used. The display with the straw does sound like a bit of blokey posturing.
There are examples, in the ballads, of the use of:
                                  a broad sword
                                  a rapier
                                  a "rousty" rapier
                                  a brand
                                  a dagger
                                  a little sword
and many, many more.(No examples of a wee pen knife so far though)
Thanks again Don. Cheers, Joy