Originates from a paraffin lamp brand 'Petromax' which was used widely in the sub-continent in case of power failure. Has now come to symbolise a man's extra-marital paramour after a much used line in a Kannada Film, courtesy Ananth Nag (who is incidentally late Shankar Nag's brother)
Kamakshi: Arre, don't you know? Preeti is Anil's petromax. If Sumati knew, she'd make chutney of Preeti and Anil!