keep one's eyes peeled

US, 19th century. Peeled probably refers to keeping one's eyelids retracted.

• "And directly below is our Mesozoic jungle river -- where, if you are lucky, you just may catch a glimpse of a very rare carnivore. Keep your eyes peeled, everyone!"
• "lookouts posted to the port and starboard bows keep their eyes peeled for the ghostly outline of approaching traffic."

• keep an eye out
• keep one's eyes open