The story
- Proto-Indo-European*oḱtṓw
- Proto-Indo-European*oḱtowós
- Proto-Germanic*ahtudô
- Proto-West Germanic*ahtudō
- Old Englisheahtoþa
- Middle Englisheightethe
- Englisheighth
- Proto-Indo-European*mon-
- Proto-Germanic*mann-
- Proto-West Germanic*mann
- Old Englishmann
- Middle Englishman
- English todayeighthman
From eighth + man, from the fact that this position is the rearmost of the eight forward positions.
Etymology from Wiktionary, CC BY-SA 4.0.