1.
Belittle, depreciate, as in They talked down the importance of the move.
2.
Silence someone by speaking loudly and persistently, as in They talked down whatever objections she brought up. [Early 1800s]
3.
Help an aircraft to land by giving directions via radio, as in The fog was so thick the control tower had to talk us down. [c. 1940]
4.
See talk down to.