Arabia Felix -> Happy Arabia
Happy Arabia (sometimes Felix Arabia) was the name previously used by geographers to describe what is now modern-day Yemen.
The southwestern corner of the Arabian Peninsula, enjoying more rainfall, is much greener than the rest of the peninsula and has long enjoyed much more productive fields.
And this explains the name: The term "Happy Arabia" is a translation of the Latin "Felix Arabia." It was only in later, classical Latin that "felix" meant "happy." Originally it meant "fertile or productive," and it was in this sense that the term was employed.