jam是果酱，tomorrow是明天，jam tomorrow含义是许诺但没有兑现的东西，可望而不可及的事物，兑现不了的许诺，空头支票。

【例句】
To get promoted by the end of the year is only jam tomorrow for him.
年底升职对他来说只不过是兑现不了的承诺。

And he was tired of friends' words of comfort and refused to settle for a promise of jam tomorrow.
唉，他已经厌倦了朋友们安慰的话语，也不满足于那些未来是美好的空话。