Dennis Johannssen has provided a good overview of the notion of negative anthropology in Adorno and Horkheimer:
From its inception, the Frankfurt School was sceptical of the new momentum anthropological thought gained during the Weimar Republic. Although its members were by and large committed to the idea of the human being’s permanent self-realization in history, which led them to reject every doctrine of man’s invariant characteristics, they nevertheless differed significantly on their willingness to integrate anthropological assumptions into their individual work. Max Horkheimer, for instance, explicitly granted philosophical anthropology an auxiliary role for Critical Theory in his essays from the mid-1930s, and he relied heavily on contemporary ethnology and anthropology in the first chapter of the Dialectic of Enlightenment.
Theodor W. Adorno, a relentless opponent of all shades of anthropological philosophy, repeatedly took issue with Walter Benjamin’s “anthropological materialism”, and dismissed philosophical anthropology tout court in his Negative Dialectics. Ulrich Sonnemann, a friend of Adorno’s and one of the last representatives of the first generation, published his main work, Negative Anthropologie in 1969. Providing “preliminary studies on the sabotage of fate,” as the subtitle reads, he called for a permanent revolution against any total theory of man. Beyond Sonnemann’s specific understanding, his notion of “negative anthropology” serves well to characterise Critical Theory’s ambiguous altercations with anthropological philosophy.