Read the passage and answer the questions that follow.
Once upon a time, I went for a week's holiday to the Continent with an Indian friend. We both enjoyed ourselves and were sorry when the week was over, but on parting our behaviour was absolutely different. He was plunged in despair. He felt that because the holiday was overall happiness was over until the world ended. He could not express his sorrow too much. But in me, the Englishman came out strong. I could not see what there was to make a fuss about. It wasn't as if we were parting forever or dying. 'Buck up', I said, 'do buck up'. He refused to buck up and I left him plunged in gloom.
What does the author mean by 'buck up'?