The first word I remember not recognizing, when I began reading on my own at about age 5, was mere. I no longer remember the title of the book, but I remember that the phrase that gave me pause was a mere mouse. I wasn’t yet in the habit of looking up words in dictionaries, so I asked my mother what it meant. She said “small,” which shut me up for the time being even though, I would later learn, it wasn’t quite right.
It’s a curious word, mere. It is not, to get this out of the way, related to motherly French mère. The OED lists multiple English definitions, including “a lake or pond” (often seen in place names such as Windermere or Woodmere) and “a siren or mermaid” (truncated from mermin, in which mer comes not from Latinate sources meaning “sea” but from Old English menon, meaning “female servant”). There’s a transitive verb to mere, meaning “to mark out” (from Old English mǣre, a boundary or border). And there are multiple adjective senses. One, whose meaning is now obsolete, meant “renowned, famous, illustrious, brilliant”; its cognates show up as parts of many European personal names, including Old High German Sigimār and Russian Vladimir. At one time mere also meant “pure,” “undiluted,” “perfect,” or “downright.”
The mere in C.S. Lewis’s Mere Christianity (1952) is to be interpreted as “absolute” or “pure,” according to scholars who know more about this subject than I pretend to. Lewis borrowed “mere Christianity” from a 17th-century Puritan, Richard Baxter.