Milking is a term with several definitions:
To give a man a handjob until he ejaculates.
To give a man a handjob until he ejaculates every last drop he has.
To give a man a handjob causing him to ejaculate more than once.
To stimulate the prostate gland, generally with one or two fingers, causing semen to flow out from the penis. This may occur without orgasm or full ejaculation.
Forcing a man to ejaculate with a handjob whether he wants to or not.
To extract milk from an animal’s udders.
To get every last bit out of a subject in conversation.
To continuously or repeatedly make fun of someone in conversation.
According to Urban Dictionary, another definition is to extract semen by giving a handjob to an unwitting roommate while he sleeps.
Extracting milk by hand, mouth or with a suction device from breasts.