ruthfully

The story

1. Proto-Germanic*hrewwaną
2. Proto-West Germanic*hreuwan
3. Proto-West Germanic*hreuwu
4. Old Englishhrēow
5. Middle Englishrewe
6. Old Norsehrygðinflu.
7. Middle Englishreuþe
8. Proto-Indo-European*pleh₁-
9. Proto-Indo-European*pl̥h₁nós
10. Proto-Germanic*fullaz
11. Middle Englishreuþful
12. Proto-Indo-European*leyg-
13. Proto-Germanic*līką
14. Middle Englishrewthfully
15. English todayruthfully

From Middle English rewthfully, equivalent to ruthful + -ly.