According to the OED the origin of the word is unascertained. It seems that the earliest recorded use of “in a jiffy” was first recorded in 1780 from The Town and Country magazine. This would predate OED’s earliest citation of 1785. Green’s Dictionary of Slang has a 1767 entry for the variant jiffin. There are a few other sources for 1767 entries.
Is there more recent information or other earlier sources?