I thought this had been explained before, though I'd forgotten the exact def. myself:
The buttie bit is easy - it just relates to the bread and butter, although where I came from a buttie would always be a single piece of bread (white sliced!) folded with a filling inside, rather than a sandwich which tends to have two slices piled on top of each other and cut. It is especially important in a chip buttie, otherwise the chips and melted butter ooze out. A large bread roll or barm cake (Manchester) is even better, although for me a chip buttie is now a luxury food reserved for very rare moments of comfort deficit.
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