Just by chance I sometimes come across a word that in nothing betrayed me it's meaning. Most of the time I have a guess in some direction, even if it proves to be the wrong one.
This one caught my eye on a random dictionary page ;

sidereal
1634, "of or pertaining to the stars," earlier sideral (1594), from Fr. sidereal (16c.), from L. sidereus "starry, astral," from sidus (gen. sideris) "star, constellation," probably from PIE base *sweid- "to shine" (cf. Lith. svidus "shining, bright").

I didn't see the starry or astral connection in any way. It looked like sidderaal, which in fact means 'electric eel'. but I knew that was nonsense. Surprised to see what it really means.