1) A slab of stone suitable for paving, or used in the plural for the paving itself.
1412 Item quod Heynton, monachus, habeat xx carectas lapidum [cart loads of stone] exceptis pavyngstones, pendants, ascheleres et flagges, York. A translated text has an earlier reference: 1404-5 ‘And for breaking 4 cart-loads of stones called … flagges in Monk Fryston quarry’. The verb to flag occurs regularly from the seventeenth century: 1648 Joseph Wood doth covenant … to remove the earth … out of the new parlor and the foundation … flaging the same, Sowerby
1706 the Bridge is flag’d and the flags breaks sumtimes, Mirfield. In 1653, the surveyors of the highways for Methley contracted with a mason for the flagging of one Hundred and Forty yards of Causey. In Beverley in 1799 the corporation subscribed 15 guineas towards flagging the market area known as the Butter Dings: it has since then been known as Butter Ding Flags.