From Diablo Wiki
To nerf (nerfing, nerfed) something in a game is to lower its hitpoints, damage or other statistic to make it less powerful. The opposite of nerfing something is buffing it.
The word nerf is an adapted word from the MMO community, much like the MMO community adapted the word "zerging" from "Zerg rush". The word itself refers to NERF-brand plasticized foam; a soft, spongy material designed specifically not to harm children. The MMO community adapted the word due to its associations with being 'safe' and 'harmless'.
In the context of the game, the word 'nerf' usually refers to changes in a patch where a class, race or other classification of the game has been lowered quality or efficiency. It could also mean that the price for something used has increased.
In some cases, "nerfed" can also refer to opponent components, such as a "boss" in an MMO.
For StarCraft, the term "nerfed" would be used if a unit got the production cost increased or their statistical numbers in terms of HP or damage lowered. In any of the Diablo games, it would be as simple as any skill variable lowered, or its cost increased.