If you're trenchant, it means you think or say smart, sharply worded things that cut right to the heart of the matter. A trenchant observation is one that makes people scratch their chins thoughtfully, or wince with embarrassment for whomever you're talking about, or both.
The word trenchant originates from tranchant, which in French means "sharp" or "cutting," and it's related to the word trench, which originally meant a line carved in wood and later came to mean a ditch carved into the earth. The word is often used to describe political commentary or cultural criticism. One person known for her trenchant wit was the author and critic Mary McCarthy, who once said of the writer Lillian Hellman, "Every word she writes is a lie, including the 'and' and the 'the'."