C. F. Keil and Franz Delitzsch on 2 Kings 19:21

2Ki 19:21 “The virgin daughter Zion despises thee, the daughter Jerusalem shakes the head behind thee.” By daughter Zion, daughter Jerusalem, we are not to understand the inhabitants of Zion, or of Jerusalem, as though בּת stood for בּנים or בּני (Ges., Hitzig, and others); but the city itself with its inhabitants is pictorially personified as a daughter and virgin, and the construct state בּת־ציּון is to be taken, like פּרת נהר, as in apposition: “daughter Zion,” not daughter of Zion (vid., Ges. §116, 5; Ewald, §287, e.). Even in the case of בּתוּלת the construct state expresses simply the relation of apposition. Zion is called a “virgin” as being an inviolable city to the Assyrians, i.e., one which they cannot conquer. Shaking the head is a gesture denoting derision and pleasure at another’s misfortune (cf. Psa 22:8; Psa 109:25, etc.). “Behind thee,” i.e., after thee as thou goest away, is placed first as a pictorial feature for the sake of emphasis.