The brain, with the frontal lobe at left in light blue, from Gray's Anatomy, via Wikipedia
The frontal lobe is the part of the brain directly behind the forehead. It's responsible for providing inhibition, essentially censoring any impulse to say or do anything, and now appears to be also related to many aspects of personality, particularly a person's sense of self.
The frontal lobe is particularly sensitive to the effects of alcohol and many other drugs, lessening its ability to inhibit speech and action, as well as increasing a person's ability to socialize.
The role of the frontal lobe was discovered after an accident to an individual, Phineas Gage, who suffered an accident where a metal bar was blown through the front part of his skull by a premature explosion. Gage survived the explosion, but the normally reserved man thereafter let loose with a string of insulting and obscene speech until his death many years later.
In Mirror Mirror, the patient's frontal lobe is damaged by an infection leading to Giovannini Mirror Syndrome where he took on the personality of the dominant person in the vicinity.
In The Social Contract, the patient's frontal lobe is damaged by an autoimmune disease, leaving him unable to stop saying anything that is on his mind.