Jargon aphasia is a strange syndrome which cause people to suddenly say nonsense words while otherwise speaking normally. Often they won't even know that they did it. And scientists are not entirely sure what that means.
Jargon aphasia arises from brain damage, either from traumatic injury or some kind of degenerative syndrome. It involves substituting in words that don't make sense while conducting a normal conversation — words that may start with the same sound, like substituting "detective" for "debt," or sometimes words that are related to each other, like substituting "bird," for "egg." Occasionally it involves substituting total nonsense words for regular words. This much most scientists all agree on.
It's everything else that causes confusion. Jargon aphasia can range from occasional word substitution in otherwise intelligible sentences to total nonsense sentences. People with the condition can be completely aware of their inability to communicate, or have no idea that they're saying words that don't make sense. As well as disagreeing about the degree of the aphasia, scientists aren't in agreement about what it means. Some conclude that jargon aphasia is a bad sign, a sign of the brain giving up on delivering the right word. Others think that it's the brain trying to get back in the game, adapting to having "lost" a word by providing substitute words or sounds that may help the person communicate.
Only a few experiments have been done examining jargon aphasia, but one of them seems to conclude that it's better to consider the condition a spectrum than a closely-defined phenomenon. A group of aphasics were asked to identify objects on cards, and repeat certain words. Unsurprisingly, the more words people missed, or replaced with nonsense words, the less they were able to realize that they were missing words. So people who used nonsense speech were unable to recognize that they were using nonsense speech. The people who only replaced a few words were able to understand what they were doing. That looks like two different conditions. But when the patients repeated the experiment, this time with some masking noise to distract them and cover up their words, those who previously were able to come up with correct answers, and recognize when they didn't come up with correct answers, suddenly were talking nonsense without realizing it. The same thing happened when the verbal tasks grew more difficult.
So it seems that the many iterations of jargon aphasia are connected, and they all depend on how hard a brain has to work to match a concept to a word and check its own answers. This extends even to neurotypical people. Nearly everyone reading this has probably, at one time or another, been distractedly talking before being informed by a friend that they said a "wrong" word without even realizing it. Jargon aphasia means having that distraction, and that struggle for words, all the time.
[Sources: Jargon Aphasia: A Confusing State of Affairs, Investigation of Self-Monitoring in Fluent Aphasia With Jargon]