So, we'll go no more a-roving
So late into the night,
Though the heart be still as loving,
And the moon be still as bright.
For the sword outwears its sheath,
And the soul wears out the breast,
And the heart must pause to breathe,
And Love itself have rest.
This metaphor is saying that the “sword” is still sharp, but the sheath in which it is kept is worn out — an erotic reference.  It can be interpreted that the desire (the sword) is still keen, but the speaker’s body has grown tired. This idea is reinforced by the following line. 
 The poem was originally included in a letter Byron wrote to Thomas Moore, in which he also noted the following: 
 …yet I find ‘the sword wearing out the scabbard,’ though I have but just turned the corner of twenty-nine. Source: Byron’s  “Letter CCLXIII. To Mr. Moore.”
So well go no more a-roving