Ducha, Spanish for “shower”, sounds unrelated to the English for the same. But it does have a less obvious cousin in English: duct; both do conduct water, towards a particular direction!
And yes, from the same root we also get, via French, douche, as in, douchebag.
Duct and Ducha both come the same Latin root, ductus, “leading”. More on that one another day.
The transformation happened due to the always-fun pattern of the -ct- words in Latin turning into -ch- words in Spanish. Thus, the d-ct in Latin and English maps almost exactly to the d-ch in Spanish.
Thus decir is another example of the “ct” sound in Latin turning into the “ch” sound in Spanish — and is also related to the English word… dictionary.
The Spanish for “night”, noche, is related (via the common Latin ancestor) nocturnal.
Here’s the interesting part: the Latin sound “ct” consistently changed to the “ch” sound in Spanish. Think “lactose” and “leche”, or “octagon” and “ocho”. And this is another example of that pattern: the “ct” in “nocturnal” is the same as the “ch” in “noche”!
Leche is a first cousin of the English lactose via a very interesting pattern: the -ct- to -ch- pattern.
Both come from the same Latin root, lactatio (literally, “suckling.”) The -ct- in that root remained unchanged as it entered English (because it entered via the sophisticated French) but that sound almost always turned into a -ch- sound as Latin evolved into Spanish. Thus the l-ct maps to the l-ch almost exactly.
The Spanish hechizo (“spell”; nothing to do with the letters in words, but what a witch casts on you!) comes from the Latin facticius (“made by art”; “artificial” — indeed, that which is artificial is just something not occurring naturally but instead made by art!).
But how did artificial change to mean “spell”? Think about it this way: casting a spell goes against nature — it’s what the wicked, crazy and profoundly unnatural woman does! Think of the three weird sisters in Macbeth, and how they unnaturally stir up all the elements!
From hechizo (more specifically, from it’s Portuguese twin cognate, feitiço), we get the English fetish. How? Well, you have a fetish when the recipient casts a spell on you to become obsessed with the object of your fetish, right? Enough said!
That root facticius turned into hechizo by changing via two common patterns: the initial F in Latin tended to turn into an H as Latin turned into Spanish (compare fig and higo, or fume and humo!) and the -ct- tended to change to a -ch- (compare noche and noctural; or ocho and octagon). Thus the h-ch of hechizo maps to the f-sh of fetish.