Worst is the superlative of bad — lower in quality than every other one. The worst film, the worst day of the year, at worst.
The set is irregular and has to be learned whole: bad, worse, worst. Worst also serves as the superlative of badly and of ill. Two useful fixed phrases: at worst introduces the least good outcome — at worst we lose an afternoon — and if the worst comes to the worst means if everything goes wrong. Note the spelling against worse: worst has the t, worse does not, and the difference is the whole grammar.
1adjectiveworstOf the lowest quality, or most unpleasant.
“That was the worst meal I have had all year.”
synonyms: poorest, most awful
2nounoutcomeThe most unpleasant part or outcome.
“The worst is over now.”
3adverbgoodIn the least good way.
“He did worst of the three and took it well.”
Too few real examples of this word to estimate which sense is commonest.
worst is a superlative.
Worst is the superlative of a word English has lost: Proto-Germanic ubilaz, bad or evil, the ancestor of evil itself. Bad, worse, worst looks irregular because worse and worst come from a different word than bad does.
Proto-Germanic
*wirsistaz
Old English
wierrest
Middle English
worste
English, today
worst
Built from
From Middle English worste, wurste, warste, werste, wirste, from Old English wierrest, from Proto-Germanic wirsistaz, superlative form of ubilaz (“bad, evil”); compare worse. Cognate with Old Saxon wirsista, wirrista (“worst”), Old High German wirst, wirsesto, wirsisto (“worst”), Danish værst (“worst”), Swedish värst (“worst”), Icelandic verstur (“worst”).
Etymology from Wiktionary, CC BY-SA