An existing word turned into an acronym by creating an apt phrase whose initial letters match the word, as to help remember it or offers theory of its origin. For example, rap has been said to be a backronym of rhythm and poetry.
Example:
One backronym familiar to every parent of a newborn is that of the Apgar score. The expansion for Apgar is “Appearance, Pulse, Grimace, Activity, Respiration.”
Actually the Apgar score is named after Virginia Apgar (1909-74), a US anaesthesiologist who developed the test in 1952 to evaluate the effects of obstetric anaesthesia on neonates.
Source for word: Dictionary.com