The phrase "old chicken" does not have a commonly used or well-known meaning. It could potentially be an idiom, but there is no established definition for it in any standard dictionaries or reference materials.
There's no better easy, healthy, delicious low calorie / points way to perk up plain old chicken and fish than to top them with a fresh fruit or vegetable salsa. (simple-nourished-living.com)
Feeling burnt - out on the same old chicken salad from the corner deli? (epicurious.com)
I took a boring old chicken salad to work for my lunch. (housegoeshome.com)