These terms were coined by T.S. Eliot in his essay "The Metaphysical Poets" (1921). Eliot saw the 17th century poets as capable of a unified sensibility which allowed them to apprehend thought sensuously.
Thought and feeling were fused: thought was felt.
Later in the century, however, a dissociation of sensibility set in which lasted all through the 18th and 19th centuries. It was only in the 20th century that a unified sensibility was regained.
According to Eliot, the 17th-century poets could "feel their thought as immediately as the odour of a rose."
In the poetry of John Donne, for example, he saw a fusion of thought and feeling. But on into the latter part of the century, with the poetry of Milton and Dryden, a dissociation of sensibility set in. Thought and feeling became disjointed, and poets became either intellectual or emotional, but not both.
Like the French Symbolists, Eliot was trying to recapture a union of emotion and intellect.