desertum

Latin deserere meant 'to abandon, to leave, to forsake' — a compound of de- ('from, away') and serere ('to join, to connect'). The past participle desertum became a noun: 'the thing that has been abandoned.' A desert was not defined by climate or geography. It was defined by the absence of people. Any place humans had left was, by Roman reckoning, a desert.

Early Christian monks seized on this meaning. The Desert Fathers of the 3rd and 4th centuries — Anthony the Great, Pachomius, Macarius — retreated to the Egyptian desert specifically because it was deserted. The theological point was inseparable from the etymology: to find God, abandon the world. The word desert carried both the physical landscape and the spiritual act of withdrawal.

Old French inherited desert by the 12th century, and English borrowed it around 1200. The meaning narrowed over centuries from 'any uninhabited place' to specifically 'a dry, barren region.' Forests can be uninhabited too, but desert lost its claim to them. Meanwhile, the related verb desert — to abandon one's post — kept the original Latin meaning intact. A soldier who deserts is doing exactly what the word says: leaving.

The connection to deserve is stranger. Latin deservire meant 'to serve well, to serve devotedly' (de- intensifying servire). Over time, 'serving devotedly' became 'earning something through service' became 'meriting something.' Deserve and desert share a root not because they mean the same thing but because Latin de- could mean both 'away from' and 'thoroughly.' One word for leaving; one word for earning. Same prefix, opposite directions.