From Proto-Germanic *fastuz; see it for cognates and further etymology.
fastr
- fast, firm, sticking fast to the spot
- close, close-fisted
- firm, faithful
- strong, hard
- “fastr”, in Geir T. Zoëga (1910) A Concise Dictionary of Old Icelandic, Oxford: Clarendon Press
- Zoëga, Geir T. (1910) A Concise Dictionary of Old Icelandic[1], Oxford: Clarendon Press