Sailing ships employed three grades of mariners before the mast (ie, apart from officers): able bodied seamen, ordinary seamen and landsmen. Landsmen were pretty useless as anything other than ballast, but the ones who learned to clap onto a lifeline when a big sea came aboard would in due course be rated OS. And as Ordinary Seamen became more proficient, able to hand, reef and steer, they would be rated AB.
No ship ever sailed with too many ABs, and many with nowhere near enough. So when a desperate captain, facing the rigours of winter in the North Atlantic or round the Horn with a fo'c'sle full of ploughmen, itinerant weavers and out-of-work miners, pleaded for help from a homeward-bound colleague, this was the reply that he most wanted to receive.
ORDINARY
