The phrase "bag of hot air" is often used to describe someone who talks excessively or without substance. There are several synonyms that convey a similar meaning, such as windbag, gasbag, blabbermouth, chatterbox, and blowhard. Windbag and gasbag both refer to someone who talks a lot without saying much of anything. Blabbermouth and chatterbox both describe someone who talks too much or too loudly. Blowhard emphasizes the arrogance or pompousness of the person in question. While these synonyms may seem insulting, they are often used humorously and can be a lighthearted way to describe someone who simply talks too much.