Cushion comes through Middle English cusshin and Anglo-French cussin. Behind them is reconstructed Vulgar Latin coxinus, formed from Latin coxa, hip.
The asterisk marks the reconstructed Vulgar Latin form. The source also gives Anglo-French quissin as an alternative spelling, not a separate chronological step.
Sources
- Merriam-Webster: cushion
Noun Word History supplies all selected forms, marks coxinus as reconstructed and glosses Latin coxa as hip.
- Online Etymology Dictionary · cushion
Meanings of the older forms on the word path.
- logeion.uchicago.edu · coxa
Meanings of the older forms on the word path.
- logeion.uchicago.edu · coxinus
Meanings of the older forms on the word path.