.. .. .. .. Yet I,
A dull and muddy-mettled rascal, peak,
Like John-a-dreams, unpregnant of my cause,
And can say nothing . .. ..

My reading of that Shakespearean quote is that in Shakespearean times John-A-Dreams was a common name for either a ghost or a dreamy, absent-minded, impractical sort of fellow. I would translate it into (slightly) more contemporary English as something like ...

Yet I, an insipid and unspirited rascal, waste away, like a ghost / a dreamer, neglecting my cause, and am dumb.

Perhaps the first interpretation reads more naturally, which would give us:

Yet I, an insipid and unspirited rascal, waste away, like a Will O' The Wisp, neglecting my cause, and am dumb.

If you can find, say, in a local library, a volume of Shakespeare or at least Hamlet, with annotations, that might give an explanation of the reference.