Fetishism is ascribing magical powers to an object. In Marx, it is used about money, commodities, work, etc. Money is the most common example: it only has power because we believe it has power, but we cannot free ourselves from it by refusing to believe in it in our society.
People often write about the Left being fetishistic, about having shied away from theory towards magical rituals. To name a few that you can read about: demonstration fetishism, picket line and union fetishism, Occupy, activism, class struggle fetishism, riots, fire and broken glass fetishism, fetishism in numbers, policy fetishism (and Safer space fetishism), Marx fetishism, standing in front of shops for full communism… Anything can be fetishised or be described as fetishism.
Opponents also ascribe such magical powers to the manifestations of the Left (belief that unions will bring about the collapse of the economy, that demonstrations will bring about the collapse of civilisation). However, Left fetishism can be more delusional than real. Whereas money can’t really be challenged, union authority can be ignored without much consequences in many cases. This leads to the label ‘fetishism’ being ascribed to anything which does not work, when it can actually stem from flawed theory, or bad execution.
Anarchism is supposed to keep away from fetishism through rational examination and the adequation of means and ends. We cannot fight alienation with alienated means, and all that. In practice, anarchists do subscribe to a lot of fetishistic attitudes. Sometimes they stem from Left fetishism: the idea that the Left will actually achieve something that we can somehow stir and make into our way to emancipation. Some anarchists think that fetishism can be empowering, or helpful, or at least completely inocuous.
If accusing someone of fetishism is seen as such an insult, it is very much because of the more usual term of sexual fetishism and the stigma related to it. Sexual metaphors are often used to describe fetishistic political attitudes “it makes them hard” “they have wet dreams about it” “they wank over it”. It is one of the rare occasion where sex negativity is wildly accepted by anarchists.