To peck is to jab or bite at something the way a bird does with its beak. A peck is also a unit of measurement, like when Peter Piper picked a peck of pickled peppers. (“Eight quarts” just didn’t sound right.)
Most birds peck at their food, and when a person eats in a bird-like way, they also peck. Another way to peck is to give someone a light kiss, a peck on the cheek. A completely different kind of peck is a unit of measurement. In the US, a peck is one quarter of a bushel, or two gallons of a dry substance. The verb sense of peck comes from 1500s thieves' slang, in which it meant "food."