What does "I have a soul of lead" mean in shakespeare?

He is a mean and unkind person and has a hard heart, is that in much ado or julius Caesar?

When he wrote this, he and his friends were out having good times, when his friend asks him why he isn't dancing. I forgot exactly why, but he says that I have a soul of lead meaning he is too heavy-hearted to be merry, or he means that as a pun, meaning I have a sole of lead, which would mean he doesn't dance well