(also Jerry Rigging) refers to makeshift repairs or temporary contrivances, made with only the tools and materials that happen to be on hand. Originally a nautical term, on sailing ships a jury rig is a replacement mast and yards (which hold the ship's rigging) improvised in case of damage or loss of the original mast.
The phrase "jury rigged" has been in use since at least 1788. The adjectival use of "jury", in the sense of makeshift or temporary, has been said to date from at least 1616 when it supposedly appeared in John Smith's A Description of New England. However, the word "jury" does not appear in the digital form of this document, as edited by Paul Royster of the University of Nebraska-Lincoln.  It appeared in Smith's more extensive The General History of Virginia, New-England, and the Summer Isles published in 1624.
There are several theories about the origin of this usage of "jury":
- From the Latin adjutare ("to aid") via Old French ajurie ("help or relief").
- A corruption of joury mast—i.e. a mast for the day, a temporary mast, being a spare used when the mast has been carried away. (From French jour, "a day".)
- Contraction in the nautical tradition for injury
While ships typically carried a number of spare parts (e.g., items such as topmasts), the lower masts, at up to one meter in diameter, were too large to carry spares. So a jury mast could be various things. Ships always carried a variety of spare sails, so rigging the jury mast once erected was mostly a matter of selecting appropriate size. Contemporary drawings and paintings show a wide variety of jury rigs, attesting to the creativity of sailors faced with the need to save their ships. Example jury-rig configurations are:
- A spare topmast
- The main boom of a brig
- To replace the foremast with the mizzenmast: mentioned in W. Brady's The Kedge Anchor (1852)
- The bowsprit set upright and tied to the stump of the original mast.
- The phrase "jerry-built" has a separate origin and implies shoddy workmanship not necessarily of a temporary nature.
- Bricolage is building from what happens to be available.
- To "MacGyver" something is to rig up something in a hurry using materials at hand, from the title character of the U.S. television show of the same name, who specialised in such improvisation stunts.
- In modern naval parlance "gundecking" (related to gun deck) tends to refer to repairs of a temporary or shoddy nature.
- In New Zealand, having a "Number 8 wire mentality" means to have the ability to make or repair something using any materials at hand (such as standard farm fencing wire).
Although ships were observed to perform reasonably well under jury rig, the rig was quite a bit weaker than the original, and the ship's first priority was normally to steer for the nearest friendly port and get replacement masts.
This site differentiates nicely: http://public.wsu.edu/~brians/errors/jerry.html
Although their etymologies are obscure and their meanings overlap, these are two distinct expressions. Something poorly built is “jerry-built.” Something rigged up temporarily in a makeshift manner with materials at hand, often in an ingenious manner, is “jury-rigged.” “Jerry-built” always has a negative connotation, whereas one can be impressed by the cleverness of a jury-rigged solution. Many people cross-pollinate these two expressions and mistakenly say “jerry-rigged” or “jury-built.”