Does anyone know why/where the name "cammando" came from for not wearing undies?

Answer 1

It's spelled 'commando' and it comes from the military unit of the same name who were notoriously tough, and also who were frequently required to go behind enemy lines with as little equipment as possible. Therefore underpants were deemed a bit of a luxury item when you were packing - and had to carry for many miles everything you packed.

Answer 2

In American slang, this phenomenon is also referred to as Freeballing (in the case of men not wearing underwear) and Freebuffing (in the case of women).

Many people go commando strictly for comfort, feeling that underwear is unnecessarily restrictive and tight, or just preferring greater mobility for the genitalia. However, in many cases there is a recognisable sexual element to the behaviour.

Going without underwear is said to have certain benefits. Dr. Caroline F. Pukall of Queens University states that going without underwear while sleeping can have significant benefits to the Vulva.[1] It is also said that tight underwear decreases sperm count in men, and it is recommended to wear more loose fitting clothing around the genitals. [2] These articles do confirm the belief that underwear can cause significant problems, which reaffirms the views of those who practice going commando.

Going commando appears to be an increasingly popular practice. A 2004 study of 7,000 people by the New York-based clothier Freshpair revealed that 9% of men and 7% of women go commando day-to-day. Those who do so semiregularly are much more numerous, possibly around 25-30%, and most people have tried it at some point.

In the United Kingdom, the adverbal use of "commando" is a relatively recent phenomenon but increasingly common. The phrase and phenomenon is often used in a declaratory way ("I'm goin' commando!"), particularly by young women to covertly create a frisson of sexual excitement or amusement amongst friends, sexual partners, etc. Modern British social mores generally recognise "going commando" as a "playful" behaviour (akin to a mild form of exhibitionism), and one that does not necessarily demonstrate an overtly sexual intent.

Notably, in Chile the act of not wearing underwear has been called "andar a lo gringo" ("go about as the americans") for decades. [3]
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More-than-likely, it is a mixing of terms that originally was refered to as being "Regimental."

"Regimental" is slang for wearing one's kilt without the benefit of underwear.