What is the origin of the idiom "stiff upper lip" and when to use it?

The origin of the idiom "stiff upper lip" can be traced back to British culture and its historical context. It is believed to have emerged during the Victorian era in the 19th century when displaying emotional restraint and maintaining a composed demeanor were highly valued traits, particularly among the British upper class. It is often invoked during times of personal hardship, loss, or tragedy, urging individuals to remain strong and composed despite their emotional turmoil.