The term "anti-solo" has taken on separate meanings from era to era. The generally agreed-upon definition of an anti-solo is a musical sequence showcasing one particular instrument that rejects traditional musical theory, advanced methods, and/or pre-meditated coordination.
An anti-solo should not be confused with an improvised solo that utilizes traditional scales and theory (either consciously or inadvertently). What characterizes an anti-solo is either a very chaotic overtone or a stripped-down bare-bones approach to musical showcasing.
The anti-solo became popular in the 1990's with the emergence of grunge acts, such as Nirvana. Nirvana heavily incorporated the anti-solo into their music, often using distortion, heavy feedback, and a non-Pentatonic/Heptatonic approach to improvising. A now-legendary anti-solo was played during "Heart Shaped Box" in a January 23, 1993 show. It consists of guitarist and lead-singer Kurt Cobain wildly strumming and picking different notes with a high-pitched feedback, creating a muddled, chaotic "solo".
At the turn of the century, the term "anti-solo" came to incorporate not only the chaos solos of the grunge era but also very simplistic, but perhaps theoretically-sound solos. The Killers' song "When You Were Young" is characterized by an anti-solo, in which a single note is repeated with little to no variation besides the occasion swing into the main riff. This classification has caused many to cite punk banks, such as the Ramones, as being some of the first groups to utilize the anti-solo.
While the anti-solo is not exclusively guitar-based, stringed instruments are its most common manifestation. Several jazz musicians, including Dave Brubeck, have been known to rely on anti-solos occasionally during live performances as a means of improvising "outside the box".
It is generally regarded by music experts and audiophiles that any kind of vocal solo cannot be an anti-solo. Distorted screaming and high-pitched wailing is viewed as a noise build but not an anti-solo, as it doesn't necessarily reject commonly-agreed-upon musical standards.
For examples of anti-solos, see:
"Scentless Apprentice" by Nirvana
"Rape Me" (Demo) by Nirvana
"Stop!" by Jane's Addiction
"Paranoid Android" by Radiohead
"I Wanna Be Sedated" by The Ramones
"Dude, what's wrong with your ears?"
"I just got out of a Classic Albums Live show. Remind me to never listen to "In Utero" at full-blast again."
"Chet, they want an encore and we're doin' Freebird! Don't screw this up for us. Just play the damn solo!"
"Bump that. I'm rockin' an anti-solo tonight."