The OED costs $100/year for US residents or £100/year for everyone else [0] but you can often get access through a library. The San Francisco Public Library has a proxy you can use if you have a library card there [1].
[0]: https://global.oup.com/academic/product/oxford-english-dicti...
Another great writing resource is etymonline.com, which gives a bare definition along with a full etymology drawn from different sources.