It should seem that these words were a kind of saying, well understood, no doubt, in those days, like proverbial sayings with us now.
Related Commentaries of Numbers 21:27
Numbers 21:27-30
27 Wherefore they that speak in proverbs say, Come into Heshbon, let the city of Sihon be built and prepared:
28 For there is a fire gone out of Heshbon, a flame from the city of Sihon: it hath consumed Ar of Moab, and the lords of the high places of Arnon.
29 Woe to thee, Moab! thou art undone, O people of Chemosh: he hath given his sons that escaped, and his daughters, into captivity unto Sihon king of the Amorites.
30 We have shot at them; Heshbon is perished even unto Dibon, and we have laid them waste even unto Nophah, which reacheth unto Medeba.