"the more distant part of the open sea as seen from the shore," 1620s, a nautical term, from off (q.v.) + noun suffix -ing (1). Outside sea-jargon, it survives in the phrase in the offing (1779) which originally meant "in the distant future;" the modern sense of "impending" developed by 1914.
there was a wedding in the offing
there was a ship in the offing
officialese
officiant
officiate
officinal
officious
offing
off-key
off-limits
off-line
off-load
off-peak