Dating a girl just to have sex. 'It' refers to a penis.
Example 1:
Laura: You and Pam are getting pretty serious, is she girlfriend material?
Joe: No, I'm just keepin it wet.

Example 2:
Mike: I heard you have two girlfriends now?
George: No, I have one girlfriend and one keepin it wet girl.
by Last Drop April 25, 2008
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