Rhythm has a very unusual spelling, breaking a lot of the common rules of thumb for spelling words. The rh is unusual; the use of y as a vowel in the middle of the word is unusual; and the lack of a normal vowel between the th and the m is extremely unusual
What is its etymology, and why does it have such an unexpected spelling?
Direct from Greek ῥυθμός (rhythmos). “rh” is not an unusual word start in Greek, “y” is just a vowel, “-os” turned into “-us” in Latin then fell off when accepted into English, so the vowel that would have been in the syllable with “m” went away.