It's such a strange adjective! I read it this morning in Ephesians 3:8 where Paul talks about the "unsearchable riches of Christ." Unsearchable means you cannot find it out; it's beyond our ability to search for or understand. (It's a rather long and beautiful Greek word: anexichniastos.)
I'm someone who searches all day long on the internet. I'm a researcher--a knowledge junkie--who loves asking questions and finding answers. I love learning new things and exhausting the information about a certain topic.
But the riches of Christ? They cannot be exhausted, searched out fully, or understood. We can only attempt, and even our best efforts at understanding represent partial examples. The very best I can imagine fails to approach the truth of it.
In a world where we search and find so well, this one concept will always elude us and be greater than our minds can handle. It won't ever end--the riches of Christ--and I can continue to search it out till my dying days, and even then, there's so much more to Jesus.