It literally means to milk. It also means a dairy cow. It could be both a noun or verb.
v. To use someone for the money he/she can give you.
n. Someone (a girlfriend, husband) that you milk for money.
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As a verb meaning to use someone for their money:
As a noun for a person being used in such fashion:
Both of these are about a specific kind of relationship, where money is given for fulfilling expectations normally seen in a romantic relationship. Essentially, this describes couples where one of the parties is rich, and the other party is after the money.
For other uses, there are options already mentioned in the comments: