The old nest is what we are always going back to. A kind of frame we are unable to get out of, that is, the tendancy or the system of values formed by our upbringing, experiences, and so on. Karmic (or conditioned) self.
Dogen also said, "Now if you wish to practice the Way of the buddhas and patriarchs, you should practice the Way of the previous sages as well as the conduct of the patriarchs with no expectation of profit; expect nothing, seek nothing, gain nothing.
Although you should quit seeking and give up expectations of buddhahood, if you stop practicing and continue engaging in your former evil deeds, you will still be guilty of seeking and will fall back into the old nest.
Without having the slightest expectation, maintain the prescribed manner of conduct. Think of acting to save and benefit living beings, earnestly carry out all good deeds, and give up former evil ones, solely for the sake of becoming the foundation of happiness for human and heavenly beings. Without stagnating in the good deeds of the present, continue practicing your whole lifetime. An ancient called this 'breaking the lacquer pail.' The Way of the life of the buddhas and patriarchs is like this." - Shobogenzo Zuimonki, 3-10
A lacquer pail is so black that you cannot distinguish between things. This is a metaphor for delusions, ignorance, ego-attachment. Breaking the lacquer pail means to get free from conditioned human sentiments.