There are various synonyms for the phrase "bad disposition" that can be used to describe someone's negative personality traits. The most common synonyms include unpleasant, disagreeable, irritable, grumpy, surly, and peevish. These words can be used to describe an individual who tends to be unfriendly, uncooperative, or displays a lack of emotional control. Other synonyms that fit the description of someone who has a bad disposition include crabby, cantankerous, corrosive, malevolent, and belligerent. In various situations, these synonyms may be more appropriate, depending on the context of the conversation or the written content being communicated.