Formal:
* Incoherent: This suggests their speech is rambling and lacks a clear structure.
* Rambling: This emphasizes their tendency to talk at length without getting to the point.
* Unintelligible: This means their speech is difficult to understand, possibly due to unclear articulation or a lack of logic.
* Erratic: This implies their speech is inconsistent and unpredictable, jumping from topic to topic.
Informal:
* Blabbermouth: This term implies they talk excessively and often reveal secrets or gossip.
* Windbag: This refers to someone who talks at length without saying anything meaningful.
* Prattle: This suggests their speech is childish or nonsensical.
* Hot air: This is a metaphor for meaningless talk.
* Nonsense: This is a straightforward way to say their speech is nonsensical.
* Drivel: This is a more harsh term for nonsensical talk.
Other options:
* Talkative: This is a neutral term that simply means they talk a lot.
* Loudmouth: This suggests they talk loudly and often inappropriately.
* Motormouth: This implies they talk very quickly and incessantly.
The best term to use will depend on the specific situation and your relationship with the person.