Definition: self-righteous; pompously moralizing
Merriam provides this commentary:
Nowadays, “sententious” is usually uncomplimentary, implying banality, oversimplification, and excessive moralizing.
It also notes, there is another meaning which seems to be at odds:
The original Middle English sense of “sententious” was “full of meaning,” a meaning adopted from Latin sententiosus. In Modern English, too, “sententious” has sometimes referred to what is full of significance and expressed tersely. Or sometimes “sententious” simply suggests an affinity for aphorisms…
In the usage I encountered below, it implies pomposity and long-windedness. But the other definition implies the opposite: short and to the point.
Why I Looked It Up
From The Covenant:
Chief Ngalo smiled. It was pleasurable to be with this old man. Always when he wanted something badly, he devised sententious and moral justifications.