It comes from caló (gypsy language). Longui = inocente.
Don't you know any word in caló?
Pinrel: foot
Fetén: good
Piltra: bed
Pirarse: go away
Camelar: to get to fall in love
etc.
These words are used commonly in Spain, though many people doesn't know they are gypsy words. I know them because my father lived in a place where there were a lot of gypsies when he was a child, and he used to say these words, so I added them to my vocabulary.