"Hadst" is an archaic word—meaning that it's a word no longer in common use—which is a contraction of "haddest," also an archaic word, which is the second person singular form of the past tense of "had."
Modern usage would be "you had." Archaic usage, such as in Shakespeare's plays and poems, would be "thou hadst"—the archaic term for "you," and the archaic term for "had."
The word "hadst" appears 107 times in Shakespeare's plays, sonnets, and major poems. Every instance of the use of "hadst" in Shakespeare's works has a "thou" in very close proximity, usually directly preceding or following "hadst."
The word "haddest" doesn't appear in any of Shakespeare's known works. Apparently the word "haddest" was archaic even in Shakespeare's time, and had been replaced with "hadst."
"Hadst" occurs six times in Shakespeare's Romeo and Juliet, and despite the textbook definition of "hadst" as "had," the use of "hadst" in Romeo and Juliet has slightly different meanings depending on the context in which "hadst" occurs.
GREGORY. (to Sampson) 'Tis well thou art not fish; if thou hadst, thou hadst been poor-John. (1.1.28-29)
It's a good thing you're not a fish, because if you had been a fish, you would have been a poor-John (a small fish of inferior quality).
NURSE. (to Juliet) An honour? Were not I thine only nurse,
I would say thou hadst suck'd wisdom from thy teat.(1.3.71-72)
An honor!? If I wasn't the only nurse you ever had, I’d say that you had sucked wisdom from your own breast.
JULIET. (to Nurse) I would thou hadst my bones, and I thy news. (2.5.28)
I wish you had my bones, and I had your news.
The nurse has been teasingly procrastinating about giving Juliet news about Romeo by complaining about how much her bones hurt.
JULIET. ...O nature, what hadst thou to do in hell
When thou didst bower the spirit of a fiend
In mortal paradise of such sweet flesh? (3.2.83-85)
Oh nature, what were you doing in hell when you put the soul of a devil in the body of a such a perfect man?
The "moral paradise of such sweet flesh" is Romeo, who just killed Juliet's cousin, Tybalt.
ROMEO. (to Friar Laurence)...Hadst thou no poison mix'd, no sharp-ground knife,
No sudden mean of death, though ne'er so mean,
But ‘banished’ to kill me—‘banished’? (3.3.45-47)
Don't you have any poison, or a very sharp knife, or something else you could use to kill me quickly which is not as demeaning and disgraceful as the word "banished"?
Romeo has just learned from Friar Laurence that he's been banished from Verona for killing Tybalt.