What is the ruling on using bad language such as "eat shit," "eat straw," "on shit," and so on?

Using offensive language is considered reviling and cursing. The general rule is that it is forbidden for a Muslim to revile his Muslim brother, due to the Prophet's (peace be upon him) saying: "Abusing a Muslim is Fusuq (disobedience to Allah)." It is permissible in some cases, such as in retaliation against someone who reviled him, provided that the wronged person does not transgress. However, refraining from it is better, due to the Prophet's (peace be upon him) saying: "The believer is not a slanderer, nor a curser, nor an obscene person, nor one who uses foul language."