Remain cheerful in a difficult situation.

This sounds like one of those rousing maxims that were drilled into the young of Victorian England - like keep a stiff upper lip. Perhaps surprisingly, the phrase is American. The first use of it that I can find is from the Pennsylvania newspaper The Evening Democrat, October 1900, under the heading Epigrams Upon the Health-giving Qualities of Mirth:

"Keep your chin up. Don't take your troubles to bed with you - hang them on a chair with your trousers or drop them in a glass of water with your teeth." - [they were easily amused in Pennsylvania in 1900].