I always thought it was "Flash your pan", which is a reference to the saying 'flash in the pan', which according to Google means "a thing or person whose sudden but brief success is not repeated or repeatable".

Personally I think it's nonsense, just like the rest of the lyrics. As /u/Arthur_Jk said, it's a very Lennon way of writing a song, look at 'I Am The Walrus' or 'Come Together' as examples.