The English word POSTERIOR is identical to the original Latin adjective, and has two different uses :
as an ADJECTIVE, it indicates that someone or something is behind another, either spatially or chronologically
it also became a SUBSTANTIVE, indicating the rear-end, especially of a person, i.e. the buttocks, as a euphemism -
DERRIERE (from the french derrière, 'behind' both as an adjective and a substantive) is perfectly analogous, but generally only used for a female bottom
See also Anatomical_terms_of_location REAR-END, often shortened as REAR, is the backside of a person, animal or object such as a vehicle. ... The buttocks are the two masses of muscle (gluteus maximus, sometimes abbreviated to glute) and fat covering the posterior aspect of the human (primate) pelvis. ... In human and zoological anatomy (sometimes called zootomy), several terms are used to describe the location of organs and other structures in the body of bilateral animals. ...
Diagram of the distribution of the cutaneous branches of the posterior divisions of the spinal nerves.
The posterior division of the suboccipital, and the medial branches of the posterior division of the second and third cervical nerves are sometimes joined by communicating loops to form the posterior cervical plexus (Cruveilhier).
Sacrales)The posterior divisions of the sacral nerves (rami posteriores)(Fig.