unrhythmical

The story

1. Proto-Indo-European*né
2. Proto-Indo-European*n̥-
3. Proto-Germanic*un-
4. Proto-West Germanic*un-
5. Old Englishun-
6. Middle Englishun-
7. Englishun-
8. Proto-Indo-European*ser-?
9. Proto-Indo-European*srew-
10. Proto-Indo-European*sru-dʰ-mo-s
11. Proto-Hellenic*hrutʰmós
12. Ancient Greekῥῠθμός (rhŭthmós)
13. Ancient Greekῥυθμικός (rhuthmikós)
14. Englishrhythmic
15. Proto-Indo-European*h₂el-
16. Proto-Indo-European*h₂élisder.?
17. Englishrhythmical
18. English todayunrhythmical

From un- + rhythmical.