Living off the fat of the land
Living well; fed by abundant crops.
In the 16th century 'the fat' meant the richest, choicest part of something. William Lambarde, in A perambulation of Kent; conteining the description, hystorie, and customes of that shyre, 1576 wrote:
" This Realme... wanted neither the favour of the Sunne, nor the fat of the Soile."
The first citation of the phrase in the 'land' version we now use is the Bible, Genesis 45:17-18 (King James Version), 1611:
And Pharaoh said unto Joseph, Say unto thy brethren, This do ye: lade your beasts, and go, get you unto the land of Canaan;
And take your father and your households, and come unto me: and I will give you the good of the land of Egypt, and ye shall eat the fat of the land.