Originally meaning “short thick block of wood,” related to Icelandic kumba. As a metaphor for someone’s intelligence, from the 1800s on. (Other insults regarding the literal density of one’s cranium: blockhead, thick, dense.)
Originally the name of a mechanical coupling device. R. F. Knucklehead, created in 1942, was a fictional character invented by the U.S. Army to show new recruits what not to do, à la “Goofus and Gallant.” His adventures were displayed in posters hung up around Air Force training fields.
Centuries ago in the rural mountain villages of France, many children grew up with an untreated thyroid condition that stunted their mental and physical growth. Their parents and neighbors referred to them euphemistically as “Christians." Some scholars say that the name was a call to remembrance of the humanity of these affected people, even though they were different. The word for Christian in the Alpine dialect was “crestin," which became crétin in modern French. Eventually, these isolated villages began to be discovered by the modern world. The word "cretin" was seized upon by psychologists around the turn of the century, borne away on a wave of science, used in academic papers for some years, and then dropped like a hot potato by modern psychology when it was used pejoratively too often and came to be seen as offensive.
(Ninth century) The word “scold” — to pelt somebody with criticism — came from the skalds, a group of Icelandic and Scandinavian poets who lounged around Viking courts composing snarky epic poems. In fact, their satire was of such dangerous proportions that “skáldskapr," literally “skaldship” — poetry — was a synonym for “libel” in old Icelandic law books. (There were similarly snarky court poets in England, called scops – which is where the word “scoff” comes from.)
Yiddish for “penis." There’s a spirited debate over where this word originated. One faction says that the word comes from the Old Polish word smok (grass-snake, dragon) and was adopted into Eastern Yiddish as a metaphor. Another faction says that it’s from the German word schmuck, which literally means “jewels," similar to the English phrase “family jewels.”
From Old French. Most likely from the phrase fils de bast, “pack-saddle child” — a child conceived upon the lumpy saddle that doubled as a bed for travelers. (Also see: the obscure word bantling, possibly from German bänkling, i.e. child conceived on a bench.)
Eight centuries ago the word had such a broad negative connotation that it was an all-purpose insult for anyone who was perceived to be careless, weak, needy, or stupid. (The French took it from the Latin – nescius, meaning ignorant, literally “not-knowing.") The word was used by each successive generation to mean something completely different — lascivious, ostentatious, slothful (1300s); scrupulous, punctilious, a dainty or picky eater (1400s); shy, not obvious or readily understood (1500s); or unimportant, trivial (1600s). To this day, it remains a good thing to say about something when you don't want anyone to know what you actually think of it.
Anna Leuchtenberger nerds out about etymology and Middle English on a regular basis, thinks in metaphors, and blogs here sometimes.