No its not the incorrect spelling of an item of clothing. SHU stands for Scoville Heat Unit. Invented by a clever man (Mr Scoville oddly enough) who established a unit by which the heat of Peppers/Chillies could be measured. I'm winging it here, but it is based on the principal that the main ingredient that gives chillies their burn is call Capsaicin. Essentially SHU establishes how many time a particle of Capsaicin (in a chilli) has to be diluted in water before the burn is no longer detecteble by the human tongue ie it ceases to burn.
Not sure how they do the test, but this is the defacto scale to which all hot chillies are measured. So if you are a Chillihead, you're going to want to understand this. The scale goes from 0 (no burn) to 16,000,000 SHU (Hottest). At its hottest (16,000,000 SHU), we are looking at Capsaicin crystals. That is pure Capsaicin extracted in a laboratory. There are no chillies on this planet that measure 16,000,000 SHU. In fact the hottest chilli known to man is just over 2,000,000 SHU at its hottest.
Also, you will notice that SHU is always given as a range and not a specific value. This is because chillies have different growing environments that can positively or adversely effect the resulting heat of a chilli. For instance, heat, sun, soil and watering can all be contributing factors that make chillies hotter or milder. So, to cover liability and not to be incorrect, SHU is always given as a guideline range.
So below is a graph that illustrates this SHU scale. To put things into perspective, a pepper has 0 SHU, a Jalapeno 2,500 to 5,000 SHU and the hottest chilli, the Carolina Reaper and average of ± 1,500,000 that means at its hottest, it could tip over 2,000,000 SHU. That is 400 times hotter than a Jalapeno. And lets face it, a Jalapeno can be very hot for a lot of folk out there.
Military peppr spray (made from Capsaicin) is about 3,000,000 SHU upward. So eating a 2,000,000 SHU chilli is pretty close to eating pepper spray. Only a real fool would attempt this :)
Some of you will say that there are chilli sauces that are in the upper reaches of this SHU scale... this is true. But the fact is, these chilli sauces have been bumped in heat with pure Capsaicin. That means these chilli sauces are not natural chilli sauces. For those chilliheads amongst us, you will say phwa - who cares! There is in my mind a fine border line between good taste and no taste. I am not one who believes making a sauce hot for the sake of the heat thrill is great. Yes I like hot chillies... but let them natural and tasty ;)