The first fruit or vegetable that begins to go off in an esky, most commonly a piece of broccoli. With a piece of broccoli, this becomes apparent as it begins to go yellow and limp. It signifies the short-lived future and imminent demise of the other perishable items within the esky, much as the death of a canary in the mines warns the miners that immediate action must be taken if they wish to live.
Ah shit, the esky canary is going yellow! Better finish off those vegies before it's too late.