Is "worser" correct grammatically?

Shakespeare also used worser in Sonnet 144:

Two loves I have of comfort and despair,
Which like two spirits do suggest me still:
The better angel is a man right fair,
The worser spirit a woman colour'd ill.

It also appears in The Taming of the Shrew and Antony and Cleopatra.

Does this mean that using worser is grammatically correct today? Not at all. Shakespeare did have a habit of making up words that precisely matched his meaning and metre when nothing suitable already existed, but in the 17th century worser was not non-standard, though it was arguably unusual. Today, it is definitely non-standard, or at the very least archaic.

In both examples, worser is used to parallel better. In Sonnet 144, the parallel is direct: the better angel and the worser spirit in the following line occupy the same position in their respective lines. In Hamlet, Shakespeare uses worser more allusively: the worser spirit contrasts with the better part or the better half (often used to describe a wife...), but that reference has to be inferred from the reader's pre-existing knowledge rather than read in the text.

It actually is in the Merriam Webster dictionary: worser.

That said, I think people will frown upon it unless you are writing to achieve an "early/archaic English" effect. They might (incorrectly) assume you are using a non-word. Also it sounds bad.

Worser is wrong. You wouldn't say "betterer".

There are adjectives already available for such purposes:

• good = better
• bad = worse

Neologism can be used to emphasize things to achieve hyperbolic connotations but quoting Shakespeare shouldn't support the idea that it is correct. Hey, but what do I know? It's not even my native language :)

• Shakespeare's language may have been 'correct' for the 1600's, but that is not a good indication for what people use nowadays (language changes).

• 'worser', as a comparative, is one of the worst solecisms in standard English, formal or informal. It is a sign of child language, where overgeneralization and mixing of multiple rules can occur.

• I'm sure there are some contexts where 'worser' might work, (others have given examples), but those instances are rare in comparison.