dimensionless

The story

1. Proto-Indo-European*dwóh₁
2. Proto-Indo-European*trísinflu.
3. Proto-Indo-European*d(w)is-
4. Proto-Italic*dis-
5. Latindis-
6. Proto-Indo-European*med-
7. Proto-Indo-European*meh₁-
8. Proto-Italic*mētis
9. Latinmētior
10. Latindīmētior
11. Latindīmēnsiō
12. Englishdimension
13. Proto-Indo-European*lewh₁-
14. Proto-Indo-European*lewHs-
15. Proto-Germanic*leusaną
16. Proto-Germanic*lausaz
17. English todaydimensionless

From dimension + -less.