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"Tinsel courage" might mean something like a fake or phony courage which doesn't really stand up to real life situations of war. See the following passage:
The youth reflected. He had been used to regarding his comrade as a blatant child with an audacity grown from his inexperience, thoughtless, headstrong, jealous, and filled with a tinsel courage. A swaggering babe accustomed to strut in his own dooryard. The youth wondered where had been born these new eyes; when his comrade had made the great discovery that there were many men who would refuse to be subjected by him. Apparently, the other had now climbed a peak of wisdom from which he could perceive himself as a very wee thing. And the youth saw that ever after it would be easier to live in his friend's neighborhood.
This is courage bourn out of bravodo and "hot air" rather than a courage from inner meaning and maturity, "with an audacity grown from his inexperience, thoughtless, headstrong, jealous, and filled with a tinsel courage."