I learned a new word this evening, while reading CP Snow's, The Light and the Dark: Lanthanine. In response to Lewis's observation that Roy was "preternaturally silent when he dined" (in college), Roy responds that "'lanthanine is the word for me.'"
What could that mean? Surprisingly, Webster's New Collegiate Dictionary (at least the 1975 edition I possess) does not include the term. A search of the Oxford Reference Online site defines "lanthanide" (today typically referred to as "lanthanoid") as any one of 15 metallic elements (atomic numbers 57-71) that are referred to in the aggregate, along with actinides, in the aggregate as "rare earth." According to the Oxford English Dictionary, they are not especially rare, but are difficult to separate from one another. It seems likely that "lathanine" is some variation on "lanthanide" or "lanthanoid," but, even so, Roy's meaning in using the term remains far from clear.
A Google search turns up no dictionary definition of "lanthanine," but only a few, botanical/chemical references. A UN Food and Agriculture Organization web page (here) uses the term "lanthanine" as an alternative to "lantadine A," a toxic substance in the leaves of a certain wild sage. Could Roy have been asserting that he is "preternaturally silent" because he is toxic or poisonous?
Finally, there is some anecdotal evidence on a linguistic blog site (here) that "lanthanine" remains in use among lawyers in the City of London, but the reference does not suggest any definition or description of how or when they use the term.
UPDATE: I believe Jonas's reference to the Greek root, in a comment to this posting, solves the puzzle. Thanks Jonas.