Racking my feeble brain cells, I seem to recall that this might have derived from RAF ground crews who used this sign indicating that they needed assistance to bring a starter mechanism to fire up an aircraft, and at the same time saying :Two six on the trolley acc". (Trolley accumulator).
Otherwise, I ascribe to Tony's description of a rude, upward sweeping gesture meaning "Up yours"?
It wouldn't be a bastardization of the Boy Scout salute, surely.
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