meaning of Trump's "stable genius"

The word stable can have many meanings. When something is stable, it's fixed and steady. If you needed advice, you'd probably go to your most stable friend, the one least likely to act crazy or be easily upset.

Whether you're talking about an object or a person, the adjective stable implies reliability and strength. You can describe a government as stable, or a relationship, or a desk. A completely different meaning of stable is the noun "building used for housing horses or other animals." Both senses of the word come from the Latin stabilis, "firm or steadfast."

In our case it is just like with the friend mentioned above.

President Trump chose the word "stable" to modify genius simply to refute the common headline that he is "unstable". Meaning roughly, whatever the media is trying to say he is - he's saying he isn't.