Possibly from the river Dart in Devonshire which enters the sea at Dartmouth, location of a Royal Navy College. Royal Navy officers who were returning to England at the end of a foreign tour of duty referred to going back to the "Old Dart" for further training.
Alternatively a variation of "old dirt" (compare old sod).
Modern English dictionary
Explore and search massive catalog of over 900,000 word meanings.
Word of the Day
Get a curated memorable word every day.
Challenge yourself
Level up your vocabulary by setting personal goals.