Antagonistic (adjective)

1. Inclined to oppose or fight against someone or something.
2. Having an opposing or conflicting action.

Origin:

Early 17th century (in the sense "opposed to each other"): from greek antagonistikos, from antagonistēs "opponent" (see antagonist).

Examples:

1. The two companies had an antagonistic relationship, constantly trying to one-up each other.
2. The young boy had an antagonistic attitude towards authority figures.
3. The relationship between the two countries was antagonistic, with both sides unwilling to compromise.
4. The new policy was met with antagonistic reactions from the employees.
5. The drug had an antagonistic effect on the patient's existing medication.