means taking small, repeated bites of food, often slowly or tentatively rather than eating a full mouthful at once

from From the verb "nibble," which surfaced in English in the 16th century. Its exact source is uncertain, but it's likely related to Low German and Dutch words like "nibbelen" — to gnaw or pick at — sharing that family of soft, repeated "nib-" sounds for small biting actions. The same root sense survives in "nib," the little point of a pen, both pointing back to a tiny, nipping tip.