It means to display courage and resolution in the face of adversity, to maintain a stoic appearance and avoid showing weakness or emotion. The idea behind it is that when deep emotions threaten to overcome a person, one of the first signs is the upper lip beginning to tremble. It seems to be a characteristically British expression, but its origin is American, where it first appeared in 1815. It was only near the end of the century that it started to appear in British publications.

Übersetzung:

Haltung bewahren

Gegenteil(e)

The opposite of "keep a stiff upper lip" is "to lose one's self-control".

Synonym(e):

You could also say: to keep up one's courage or to keep one's chin up.