The story
- Old English · inherited fromfǣtnes“fatness, the richest part of anything”
- Middle English · inherited fromfatnesse
- English todayfatness
From Middle English fatnesse, fattenesse, from Old English fǣtnes (“fatness, the richest part of anything”), equivalent to fat + -ness.
Etymology from Wiktionary, CC BY-SA 4.0.