In the final stanza of this poem, the speaker asks his beloved not to allow her heart to imagine, or divine, some ill that may befall him while he is away from her; otherwise, if she does, he fears that "Destiny" might take its cue from her thoughts and fulfill her fears for him. In other words, if she imagines bad things happening to him during or after his departure from their home, she could actually, inadvertently, cause those bad things to happen.

Have you ever heard of a self-fulfilling prophecy? This is when you think that something is going to happen so much that you unwittingly actually cause it to happen. For instance, someone might tell you to think positively before going in to take a big test because thinking negatively—believing that you will do poorly or that you didn't study enough—could actually lower your confidence and accidentally cause you to do worse than you otherwise would have. This is similar to what the speaker suggests: that his beloved could actually help to bring about some "ill" for him in his journey away from home by simply thinking about or believing that something bad will happen.