Stewart Gilligan Griffin, more commonly known as Stewie, is a fictional character in the animated television series Family Guy.
Seth MacFarlane has established a few general rules as to how Stewie communicates with others.
Brian Griffin always understands:
Brian is, as of now, the only main character whose ability to understand Stewie is unambiguous; the two always converse normally. This may be because they are two characters that it doesn't make sense for them to talk. In "Running Mates", Brian briefly appears to hear Stewie's thoughts; this phenomenon has not been explored in subsequent episodes.
Adults understand, but disregard Stewie:
Throughout the first two seasons of the series, it was suggested that none of the other prominent characters (with some minor exceptions) could understand what Stewie says, apparently hearing what he says through their ears as infantile blathering. At the end of the episode "E. Peterbus Unum", the question of whether adults could understand Stewie is raised.
In the DVD audio commentary, MacFarlane explains adults can understand him, but don't take him seriously, "sort of like... if a four-year-old who [could] talk told you to 'f*** off', you'd think it'd be cute." In this manner, characters acknowledge Stewie, but pay no mind to his often-insulting addresses, such as calling Peter "the fat man" and referring to Lois by her given name.
His insults are occasionally heard, such as when Stewie tells Lois "Why don't you burn in hell?!" Lois replies, "Well, no dessert for you, young man."