Pickling a cucumber stores and preserves it for later use, so the term is used for serialization. Still an odd analogy though, since you can't unpickle a pickle. I've heard freeze/thaw which is at least a little better.

Pickle is the Python module that (de)serializes Python objects to some sort of binary format. Failure to pickle means the object couldn’t be serialized — the most common instance is probably trying to pass a lambda to a multiprocessing worker, because they don’t have names (you should use a def or functions.partial instead).