In the offing
Imminent; likely to happen soon.
This is one of the many phrases of nautical origin. It is quite simple to understand once you know that 'the offing' is the part of the sea that can be seen from land, excluding those parts that are near the shore. Early texts also refer to it as 'offen' or 'offin'.
Someone who was watching out for a ship to arrive would first see it approaching when it was 'in the offing' and expected to dock before the next tide. Something that is 'in the offing' isn't happening now or even in a minute or two, but will inevitably happen before long. The phrase has migrated from its naval origin into general use in the language and is now used to describe any event that is imminent. 'The offing', although more usually used in the context of ships arriving, derives from the adjective 'off', which was used since at least the 14th century to mean 'away from' - as in 'casting off', 'setting off', 'be off with you' etc.
In its literal nautical sense, the phrase has been in use since the early 17th century and the earliest citation of it that I have found is a quotation from Samuel Purchas's Purchas his Pilgrimes, which was published in 1625. Purchas printed the account of Sir Samuel Argall (called Argoll in some sources), the English naval officer who had set off from Jamestown, Virginia, on 19th June 1610, looking for a route to Bermuda:
The nine and twentieth [29th June 1610] in the morning I weighed [anchor] againe, the wind being all Southerly, turned until night, and then I came to an anchor in seven fathomes water in the offing to Sea.
The phrase wasn't commonly used until the beginning of the 18th century, as in this example from Josiah Burchett's Memoirs of Transactions at Sea During the War with France, 1703. This is, incidentally, a classic example of the use of the long form of the letter 's' in 18th century printing (rather inexactly represented by a lowercase 'f'):
...fome other fmall Ships were feen in the Offing. Thofe Ships ftood away with their Boats a-head, fetting fire to fome, and deftroying and deferting other of their fmall veffels.
All of the 18th century citations of 'in the offing' refer to the offing as a physical place. It wasn't until the mid 19th century, in America, that our presently understood figurative meaning began to be used. An early example of that comes in S. B. Beckett's Portland Reference Book and City Directory, 1850:
We have known wives to forget that they had husbands..., especially when they supposed that a tax bill or a notification to do military duty was in the offing.
See other Nautical Phrases.