An Etymological Dictionary of the German Language/Annotated/fest
fest, adjective, ‘firm, solid, strong,’ from Middle High German vęst, vęste, Old High German fęsti, adjective, ‘firm, strong, steadfast’; see the corresponding adverb fast, which is not mutated; neither was the adjective originally formed by mutation, since, according to Old Saxon fast, Anglo-Saxon fœst, English fast, Old Icelandic fastr, adjective, ‘firm,’ we have to assume a Gothic *fastu-, which is probably an old to- participle like laut, traut, zart, alt, &c., from the root fas-, ‘to fasten’; *fasta-, literally ‘fastened,’ then ‘firm.’ Gothic still retains only the verb fastan, ‘to keep firm, hold fast.’ See fasten.