eye for/to the main chance, to have an

To look out for one’s own best interest  or  for  the  opportunity  that  will  be  most  profitable. The  term  is thought to come either from betting in cockfights (main being an obsolete word  for  cockfight)  or  from  a  game  of  chance  called  hazard, in  which  the main  signified  the  number  (anywhere  from  5  to  9)  called  by  the  caster before  the  dice  were  thrown  and  chance  signified  the  second  throw  of  the dice, which  determined  the  total. In  any  event, the  expression  dates  from the  sixteenth  century, was  recorded  by  John  Lyly  and  Shakespeare, among others, and appeared in John Ray’s English Proverbs of 1670.