dialectally

The story

1. Proto-Indo-European*dwóh₁
2. Proto-Indo-European*trísinflu.
3. Proto-Indo-European*dwís
4. Ancient Greekδιά (diá)
5. Ancient Greekδῐᾰ- (dĭă-)
6. Proto-Indo-European*leǵ-
7. Proto-Indo-European*léǵ-e-ti
8. Proto-Hellenic*légō
9. Ancient Greekλέγω (légō)
10. Ancient Greekδιαλέγομαι (dialégomai)
11. Ancient Greekδῐᾰ́λεκτος (dĭắlektos)
12. Latindialecto
13. Middle Frenchdialecte
14. Englishdialect
15. Proto-Indo-European*h₂el-
16. Proto-Indo-European*h₂élisder.?
17. Englishdialectal
18. Proto-Indo-European*leyg-
19. Proto-Germanic*līką
20. English todaydialectally

From dialectal + -ly.