Learn English – Why the “top” in “top hat”

It refers to the time of youth when one was naive and inexperienced, and therefore happy and optimistic - in other words, when one was "green," as in "unripe" or "not yet mature." It's a bit of a convoluted pun.

Like so many English idioms, the term was coined by Shakespeare in the 17th century (Antony and Cleopatra.)

It’s a British expression, now used quite generally of university study, that used to be especially common at Oxford and Cambridge. I suspect that it reflects the way in which education was traditionally organized at those universities: a series of set-piece lectures, not necessarily compulsory, and a great deal of independent reading that was regularly discussed with a tutor, either individually or in small seminars. However important the lectures were (or in some cases perhaps weren’t), the bulk of a serious student’s time and energy went into reading, digesting what was read, and writing based on that reading, especially in subjects like history, literature, and philosophy. In this setting reading history (say) would be a natural pars pro toto.