Against is a preposition.

What is an adjective for against?

The word anti comes from the prefix anti-, which means “against” or “opposite,” and is still used in English words, such as antibodies and anti-hero. As a word on its own anti is an adjective or preposition describing a person or thing that is against someone or something else.

What is this word against?

preposition. in opposition to; contrary to; adverse or hostile to: twenty votes against ten; against reason. in resistance to or defense from: protection against burglars. in an opposite direction to: to ride against the wind.