batten

batten² (BAT-en) - n., thin strip of wood used in construction to hold members of a structure together or to provide a fixing point; (Naut.) a thin strip of wood inserted in a sail to keep it flat, a strip of wood or metal used to hold the tarpaulin covering a hatch in place; (Theat.) a length of metal pipe hung from the gridiron, for suspending scenery or equipment, as drops, flats, or lighting units. v., to furnish or support with battens, spec.: (Naut.) to use battens to fasten the edge of material covering hatches (usually followed by "down"), (Mach.) to fasten (work) to a table or bed before machining it, (Theat.) to suspend from a batten, and so on.


So to batten down the hatches (the whole point of looking this up) is to cover the hatch with a tarp and then use thin strips to wedge the tarp tight, and so make it watertight against the waves washing over deck. In this sense (as opposed to batten¹, meaning to fatten up) in use since the 1400s, from bataunt, batent, finished board, from Old French batant, past participle of batre, to beat, from Latin battuere, to beat.

---L.