Not your fault of course but it was always a stupid name. It isn’t arrested or inhibited, during a stage of development, resulting in an underdeveloped outcome (retarded). Like a fire retardant door stops the fire developing, as it would usually on doors. In the case of this drug, the release is inhibited, as its, presumably, a pro-drug.
They could have called it “long lasting”, “pro-drug”, “pro”, “inhib” or “slow release” and these would have all been accurate descriptions. However, retarded isn’t accurate. They chose it anyway though.