What does "jalap" mean?

Jalap: The tubers of the Mexican plant Ipomoea purga (or Exogonium purga), a climber much like the morning-glory. The abstract, extract, and powder, prepared from the tubers, are well known purgative medicines. Other species of Ipomoea yield several inferior kinds of jalap, as the I. Orizabensis, and I. tuberosa. False jalap, the root of Mirabilis Jalapa, four-o'clock, or marvel of Peru.