1) The word was in use in Old English but it has a wide range of meanings and a very complicated etymology. It could refer to a thin piece of timber, longer than it was broad, and generally thinner than a plank, as in the compound term floor-board.
1296-7 pro … bordis, meremio emptis cum servicio sarratorum
1318-9 Pro grangia de Rither cooperienda cum bordis, Bolton Priory. A good deal of furniture was made of boards, especially tables and it came eventually to mean table: 1489 j burde with a payre tristes, Yarm
1581 1 longe bord, 1 lytyll bord, 1 longe forme, 1 lytyll round table, Anston
1589 one table borde one binke borde, South Cave. This essential timber was already being imported during the Middle Ages
1354-5 In xiij bord de Estriche, Ripon.
2) The boards in a coal-mine were passages or working-places from which coal had been taken, cut at right angles to the line of cleavage of the coal.
They were called workings, boards or headways in The Compleat Collier. One early reference is to 2 yards of board in Farnley, in 1690, and later entries in the same accounts include: 1716 levell board 35 yards
1718 straitboard 2 yards. A Sharlston workman testified in c.1700 that he … did lately drive and worke two boards or workings in a pitt … so exceeding much above the usuall and accustomed breadth and in severall places did leave a little or no pillars to support the said work. The phrases ‘board and wall’ and ‘board and pillar’ are references to methods of working coal in which ‘pillars’ of coal were left uncut.