"Dim" because they are not smart enough to realize that their actions are hurting their bf/gf. "Dick" because their dimness results in actions that are also done by dicks.

Something to call someone you like but who treats you like shit, they usually are good friends and fun to hang around but are also low-key assholes.
Oh my god! I can't believe I fell for such a dim dick! He is never going to realize how poorly he treats me!
by kb+ewok5e July 22, 2018
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