Alas, on that beloved's promise of tomorrow, we ended up spending our entire life just waiting.

Here, the poet calls the beloved a 'but'—a stone idol who is exquisite to look at, but completely cold and indifferent inside. The beloved made a casual promise of meeting 'tomorrow,' and the lover spent an entire lifetime waiting for that tomorrow to arrive. It is a tender yet heart-wrenching picture of devotion, showing how a lover's hopeful innocence turns a single fleeting promise into a lifelong, tragic exile.