"strip of wood, bar nailed across parallel boards to hold them together," 1650s, Englished variant of baton "a stick, a staff" (see baton). Nautical sense "strip of wood nailed down over a tarpaulin over a ship's hatches to prevent leakage in stormy weather" is attested from 1769.
"to improve; to fatten," 1590s, probably representing an unrecorded Middle English dialectal survival of Old Norse batna "improve" (source also of Old English batian, Old Frisian batia, Old High German bazen, Gothic gabatnan "to become better, avail, benefit," Old English bet "better;" also see boot (n.2)). Related: Battened; battening.
"to furnish with battens," 1775, from batten (n.) "strip of wood, bar nailed across parallel boards to hold them together." Nautical phrase batten down "cover (hatches) with tarpaulin and nail it down with battens to make it secure" is recorded from 1821. Related: Battened; battening.