1. Concealed or hidden; complex, obscure, and difficult to understand: "The poetry of Harry Philiips was often described as abstruse and, unfortunately, it was often relegated to the bottom shelf in the library."
2. Difficult to penetrate; incomprehensible to one of ordinary understanding or knowledge: "Some scientists may understand Einstein's theory of relativity; however, for most nonscientists, it continues to be an abstruse collection of surrealistic or unrealistic ideas."
4. Etymology: from Latin abstrusus, past participle of abstrudere, "conceal"; literally, "to thrust away", from ab-, "away" + trudere "to thrust, to push".