"They shut me up in Prose-" consists of three quatrains, which are stanzas of four lines each. In the first stanza, the speaker uses a metaphor to liken being forced to think in prose—to think literally and rationally—to being locked in a closet as a girl to learn to be still. Using "prose" is equated to being restricted, confined, or imprisoned as a way to learn to be docile and obedient ("still").
In the second stanza, the speaker scoffs at the idea that being confined in a small space can force her mind into obedience. By repeating the word "still," and with exclamation, she shows how confining her to keep her immobile doesn't work. The speaker goes on to jeer at the idea that keeping her body still kept her mind still, saying that her brain continued to go "round." She states they could as easily have kept her docile by imprisoning her as they could have "pound[ed]" or imprisoned a bird. Like a bird, she simply slipped away between the iron bars of the cage. In her case, she slipped away by using her imagination.
The speaker moves to the male gender in stanza three, beginning the stanza with "Himself." Himself, a male, can easily impose his "will" on a female, look down on her captivity, and "laugh." However, the speaker ends on a note of subversion that sustains the defiant tenor of the first two verses, stating, "No More Have I." By this, she means she, too, has laughed from afar at her girlish captivity just as a man might, because she wasn't a captive at all.
Dickinson causes abrupt pauses with her dashes, underscoring the subversive theme of the poem, in which a woman declares her freedom from the restrictions on thought that patriarchal "prose" and society imposes on her.