John Milton coined the phrase 'silver lining' in his poem Comus: A Mask Presented at Ludlow Castle, 1634:
I see ye visibly, and now believe
That he, the Supreme Good, to whom all things ill
Are but as slavish officers of vengeance,
Would send a glistering guardian, if need were
To keep my life and honor unassailed.
Was I deceived, or did a sable cloud
Turn forth her silver lining on the night?
I did not err; there does a sable cloud
Turn forth her silver lining on the night,
And casts a gleam over this tufted grove.
It is a metaphor comparing the silvery, shining edges of a cloud backlit by the Sun or the Moon to an unseen silver lining for the back of the cloud.