This word comes from the Latin “sapo,” meaning “soap,” plus the adjective-forming suffix “-ous.” “Saponaceous” describes substances used to make soap, such as aloe vera gel and glycerin, but it can also describe substances that have a soapy feel or appearance, like mica and some shales. Around the 19th century, the word had a brief stint of being used in a metaphorical sense to describe people who exhibit an evasive, slippery character.