An Etymological Dictionary of the German Language/fest
fest, adj., ‘firm, solid, strong,’ from MidHG. vęst, vęste, OHG. fęsti, adj., ‘firm, strong, steadfast’; see the corresponding adv. fast, which is not mutated; neither was the adj. originally formed by mutation, since, according to OSax. fast, AS. fœst, E. fast, OIc. fastr, adj., ‘firm,’ we have to assume a Goth. *fastu-, which is probably an old to- partic. like laut, traut, zart, alt, &c., from the root fas-, ‘to fasten’; *fasta-, lit. ‘fastened,’ then ‘firm.’ Goth. still retains only the verb fastan, ‘to keep firm, hold fast.’ See fasten.