monomaniacal

The story

1. Proto-Indo-European*men-
2. Proto-Hellenic*mónwos
3. Ancient Greekμόνος (mónos)
4. Ancient Greekμονο- (mono-)
5. Englishmono-
6. Proto-Indo-European*men-
7. Proto-Indo-European*mn̥yétor
8. Proto-Hellenic*məňňómai
9. Ancient Greekμαίνομαι (maínomai)
10. Ancient Greekμᾰνῐ́ᾱ (mănĭ́ā)
11. Ancient Greekμανιακός (maniakós)
12. Late Latinmaniacu
13. Frenchmaniaque
14. Englishmaniac
15. Proto-Indo-European*h₂el-
16. Proto-Indo-European*h₂élisder.?
17. Englishmaniacal
18. English todaymonomaniacal

From mono- + maniacal.