Jiffy (physics). A jiffy, as defined by Edward R. Harrison, is the time it takes light to travel one fermi (one femtometer), which is about the size of a proton.
Smoot. The smoot is a unit of length created by a bunch of (probably drunk) MIT students in 1958. The unit is named after George Smoot, and was used to measure the lenght of the Harvard Bridge. One smoot is equal to George Smoot’s height at the time (five feet and seven inches). To measure the length of the bridge, Smoot repeatedly lay down on the bridge, allowed his fraternity brothers to mark his position, and got up again. The bridge was measured as “364.4 smoots plus or minus one ear”. The markings on the bridge are still there, as they are repainted each semester. Appropriately, George Smoot later served as Chairman of the American National Standards Institute (ANSI) and President of the International Organization for Standardization (ISO).
I like “furlongs per fortnight”. Mostly because of the alliteration. Other than that, it’s probably a useless unit of velocity unless maybe if you’re talking about the growth cycle of some sort of creeping vine or slug racing.
Acre-foot, a unit of volume which is cuboidal with all three dimensions different: one foot by one chain by one furlong.
I personally would be seriously alarmed by a creeping vine that was growing at a rate of one furlong per fortnight, which is 110 yards per week, or about two feet every hour - just about fast enough to be clearly visible.
The OP already beat me to the Smoot; I’ve seen the markings on the bridge many times. I would add that Oliver Smoot’s cousin George Smoot is a $1,000,000 quiz show winner, appeared on an episode of The Big Bang Theory, and won the Nobel Prize in Physics. And Google’s conversion engine is aware of the Smoot, and can convert to and from other units of distance.
I’ve heard it said that the attoparsec per microfortnight is approximately equal to 1 inch per second. While googling to confirm that, I found this page of whimsical units of measurement. It explains that 1 furlong/fortnight = 10 Snail’s Pace, and that the speed of light is 18 terasnailpaces.
That is the origin of the term dramshop which in the US is a legal term meaning a bar or tavern since "[t]raditionally, it referred to a shop where spirits were sold by the dram…"
Another one that I’ve often found useful in daily life is the millihelen, which is the quantity of beauty needed to launch one ship.
Also, the sheppy: “Measure of distance (equal to approximately seven eighths of a mile), defined as the closest distance at which sheep remain picturesque”, according to the Meaning of Liff.
I love some of the old customary units of measure from Spain. The only “hiccup” with them is that they tended to have different values according to the region…
Two of them, for some reason, are particularly close to my heart:
ARROBA (whose abbreviation/symbol, btw, was “@”) – a unit of weight that was used specifically for livestock – most often for pigs. It ended up being defined as 25 Castilian Pounds (or, in metric units, roughly 11.5 kg), but originally it was 1/4 of a quintal, the quintal being defined as “the load that a donkey or a mule could carry” (the word comes from the Arabic “ar-rub”).
ALMUD – a unit of volume for grain and other dry objects. Its value varied wildly from one region to the next; in Castile it was roughly 4.625 liters (the word comes from the Arabic “al-mudd”).
