How did the word gay come to mean homosexual?

The word gay is from the Old French gai, meaning “merry.” It came to
mean reckless self-indulgence in the seventeenth century, and it was-
n’t until the 1930s that its homosexual connotation came out of the
prison system, where the expression “gay-cat” meant a younger, inex-
perienced man who, in order to survive, traded his virtue for the pro-
tection and experience of an older convict.