* It depends on the context: Do you mean the opposite in terms of its physical nature, its presence/absence, or its function?
* Physical nature: You could argue the opposite of hair's texture is smooth skin, or the opposite of its structure is something solid like bone.
* Presence/absence: The opposite of having hair could be being bald or having no fur.
* Function: Hair's function is protection, insulation, and sometimes sensory input. The opposite could be something that doesn't offer those things, like bare skin or a smooth surface.
Ultimately, the best opposite for "hair" depends on what you're trying to say.