"Heehaw" is a word that usually describes the sound made by a donkey or a mule. However, when used in informal conversations, it can have other meanings, such as laughing out loud or making fun of someone. If you want to use a different word instead of "heehaw" to describe a donkey's braying sound, you can use synonyms like "bray," "neigh," or "whinny." As for the informal use of "heehaw," you can substitute it with words like "guffaw," "chuckle," or "snicker." Regardless of which word you choose, remember that context is key in determining the meaning of a word and using it appropriately.