What does Genesis 39:18 mean?

Potiphar's wife concludes her false accusation against Joseph (Genesis 39:7–17). She claims Joseph took off his cloak and attempted to rape her, so she screamed, and he ran away. Whether or not Potiphar then spoke to Joseph, or anyone else, we are not told. The following verse suggests that he did not, and that his reaction came only after hearing her words (Genesis 39:19).

On one hand, this will be a difficult lie for Potiphar to see through. Joseph was likely seen running from the house without his cloak. The servants heard the scream and then heard the story—albeit a false one—straight from Potiphar's wife. A young, attractive, successful slave might well have let his position go to his head and attempted such a thing.

The expected response would be for Potiphar to have Joseph executed immediately, if not to kill the slave himself. On the other hand, Potiphar's wife has been extremely aggressive in her attempts at seduction. And Potiphar gave Joseph a restriction which otherwise would seem to go without saying: that the master's wife was off-limits. That, and Potiphar's relatively merciful punishment (Genesis 39:20), suggests he might not have fully trusted his wife's version of the story.