What kind of a lousy word is this? It bugs the eyes! It’ll have you pulling your hair out! It starts with such a cluster of consonants, and then it has a small infestation of dots. You can see as is at the end, but do you want to take it as is?
You don’t want to take what it names as is, I assure you of that. Here’s the Encyclopædia Britannica definition from the 1768 edition, courtesy of the Twitter feed @Britannica1768: “PHTHIRIASIS, in medicine, the pedicularis morbus, or lousy disease, is most incident in children, though adults are not wholly exempt.”
If that doesn’t make clear sense, I should amplify: although nowadays we use lousy always to mean simple ‘bad, of poor quality’, it actually first – and still literally, for those who know – means ‘infested with lice’. That’s what pedicularis morbus means. But why use straightforward English if you can use a peculiar – or particularly ridiculous – term?
I won’t say that phthiriasis is somehow reminiscent of scratching or of the sound of shears and razor clearing the hair from your head (the best way to rid yourself of lice, although many people so dread shaving their heads they would rather apply highly toxic treatments instead). To me it actually seems more like psoriasis (another skin problem, but not contagious) mixed with the sound you make when trying to get a hair or seed off your tongue, “fth.” And yes, if you’re wondering about how it’s said, it’s /θᵻˈraɪəsəs/ or /θaɪˈraɪəsəs/ (“thi-RYE-a-sis”) or, if you want to be truly nitpicky about it, the same with /f/ before the /θ/.
And whence comes this word? Via Latin from Greek, derived from ϕθείρ ftheir ‘louse’. That in turn is most likely from ϕθείρειν ftheirein ‘destroy’ – not because having lice destroys your life (it can make rather a mess of it) but because lice were believed to be generated spontaneously in decaying flesh.
It’s a long word for such a small beastie: one big foot of four syllables – extra ironic because the Latin name for one genus of lice (the one you’ll most likely find in your hair) is Pediculus, which comes from Latin for ‘little foot’. Well, you’ll have lots of little feet, anyway.