the GNU version of the Collaborative International Dictionary of English
adj. Resembling or containing gypsum; partaking of the qualities of gypsum.
The Century Dictionary and Cyclopedia
Of the nature of gypsum; partaking of the qualities of gypsum; resembling gypsum.
In botany, very dull grayish-white.
Word Usage
"The oldest is the Gehannam Formation (ca 40-41 million years old) consisting of white marly limestone and gypseous shale and yielding many skeletons of archaic whales (archaeocetes), sirenians (sea cows), shark teeth, turtles, and crocodilians."