What's the meaning of the phrase 'Pass away'?
Euphemism for dying.
What's the origin of the phrase 'Pass away'?
'Pass away' must be one of the oldest euphemisms known in English. It was coined at a time and place, that is, the 15th century in England, when most people would have believed that the departing of the soul of a dead person was a literal physical event. Indeed, 'passing away' didn't mean dying as it does now. When wakes were held for recently deceased people the attendees believed that the dead person could hear and comprehend everything that was being said: it was only later, when the funeral rites were complete, that the dead person 'passed away' and began the journey toward either Heaven or Hell.
As such, 'pass away' wouldn't have been considered euphemistic but merely a literal description of events.
The Lay Folks Mass Book, circa 1400 has an very early of the use of 'passed away', in Middle English:
Graunt... rest & pese... to cristen soules passed away.
[Grant rest and peace to Christian souls passed away]
See also: 'Pass over to the other side'.