Why do we say "Keep your chin up"?

Meaning:

Always remain cheerful in the face of problems

Background:

Although one would have thought that such an expression would have been coined centuries ago, probably by some robust British military type addressing either his children or his troops, it appears that the earliest known use of this phrase, sometimes shortened to 'chin up', is from the USA.

The website phrases.org.uk cites a reference in the Pennsylvania newspaper, The Evening Democrat, from October 1990 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."

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