The term was used by the anthropologist Clifford Geertz in his The Interpretation of Cultures (1973) to describe his own method of doing ethnography (Geertz 1973:5-6, 9-10). Since then, the term and the methodology it represents have gained currency in the social sciences and beyond. Today, "thick description" is used in a variety of fields, including the type of literary criticism known as New Historicism.
In Geertz's essay, "Thick Description: Toward an Interpretive Theory of Culture", (Geertz 1973:3-30) he explains that he adopted the term from philosopher Gilbert Ryle. Ryle pointed out that if someone winks at us without a context, we don't know what it means. It might mean the person is attracted to us, that they are trying to communicate secretly, that they understand what you mean, or anything. As the context changes, the meaning of the wink changes.
Geertz argues that all human behaviour is like this. He therefore distinguishes between a thin description, which (to extend our example) describes only the wink itself, and a thick description, which explains the context of the practices and discourse within a society. According to Geertz, the task of the anthropologist is to give thick descriptions.