Nabki comes from bakā–yabkī and is a first-person plural imperfect: “we weep.” Here it functions as a jussive response to a request following the imperative qifā: “Stop, and then let us weep.”
In the jussive form the final weak letter yāʾ is dropped. The ordinary indicative is nabkī, whereas the verse has nabki. The short form encodes a grammatical relationship as well as the poet's urgency.
It is not simply a statement that “we are crying”; the poet asks for a pause that will make mourning possible.