- contagion: A harmful, corrupting influence
- imbodies: assumes the qualities of a material body
- imbrutes: To make or become brutal. (American Heritage Dictionary)
When trying to understand a quotation in poetry, the first place to start is with the definitions of words. Look up the meaning of words that are unfamiliar or are used unfamiliarly. Three words in this quotation that justify looking up are "contagion" "imbodies" and "imbrutes."
The second step is to sort out any rhetorical variations like sentence structure that inverts or otherwise rearranges standard English word order of Subject Verb Object (of the verb) or Complement (of the subject) or imposes other sorts of variation. The last line and a half are constructed as a rhetorical variation in that there is employed an archaic verb construction:
till she quite loose
The divine property of her first being
"Loose" in this usage is a transitive verb (one that requires an Object: loose what?) that means to release, undo, or discharge. In these lines, "she" is the "soul" of the earlier line: "The soul grows clotted." "Clot" is an unpleasant word meaning to form a thick lump or mass (American Heritage).
Let's put the definitions together in a paraphrase of these lines.
- When "lewd and lavish act of sin, / Lets in defilement," the spiritual, formless soul develops a gooey mass that becomes a physical entity, an embodied material lump, that makes the soul brutal, mean, vicious until the soul entirely loses the "divine property of [its] first being," of its spiritual essence, of it true and original nature.
What this means is that sexual acts of sin corrupt and damage the soul to the extent that the soul becomes unrecognizable, is changed from spiritual and formless, and becomes brutal, mean, hostile and vicious.