It’s not the humping his guitar bit, although that contributes; it’s the start-with-Chuck-Berry-licks-move-to-Hendrix-and-Van-Halen-(badly)-licks-and-then-stop-and-realize-you-lost-the-rest-of-the-band-and-audience bit.
Both. He learned it from Pete in the original timeline, who learned it from Marty in the modified timeline. The same thing happened to Chuck Berry.
Yeah, my nomination of shredding doesn’t apply here - he’s not playing all that fast or doing a ton of notes. It’s just wanking.
Lack of talent.
Feeling up the G string.
You know, I was never really sure what to make of that. Is it really offensive? I can only gather that since Marty learned it from Chuck Berry, then Berry had to have been the originator. Marty wouldn’t have known it otherwise, so the scene simply takes place before Berry had conjured his innovation. Right?
There are any number of ways one can hand-wave it away, and it is a silly little moment in a fun, silly movie. I have no substantive reason for my beef…
But, alternatively, to insinuate that some 80’s suburban white boy Terminator’d back in time to influence Chuck Berry just rankles me (speaking as a late 70’s/early 80’s suburban white boy myself). What Chuck Berry did to build on T-Bone Walker and Louis Jordan, adapting horn lines from jump blues to fit into a country/blues/boogie rhythm - and cross over to be one of the first Brown-Eyed Handsome Men to make it in mainstream music right at the birth of rock? To be THE genesis of rock n’ roll licks as we know it, per John Lennon’s famous quote and Keith Richards’ famous adulation, and be held out as a cute little reference in a movie like that - I dunno, it just bugs me.
I have no idea why this example bugs me more than, say, Bill and Ted kidnapping Socrates by dropping quotes from bad soap operas (“like sands of the hourglass…”), but it does…
Showmanship.
Guitarists aren’t the only ones to do it: This Drummer Is At The Wrong Gig - YouTube
Yeah, well, at least he didn’t rape his mom, or perform his own skateboarding stunts.
In his song “I Can’t Drive 55”, Sammy Hagar calls it “metal mastrubation”.
I don’t have a cite, and I’m pretty indifferent to Hagar, but I’ve always liked this phrase.
I’m with shredding as a term.
Also +1 about Chuck Berry – “his” pianist Johnnie Johnson deserves a hell of a lot of the credit for those classic tunes, even though he never got any AFAIK. And that’s a cat who didn’t even like playing in B-natural, according to his Homespun concert with Jimmy Vivino helping out on guitar – all that stuff lies fine in Bb, but everybody plays “Johnny B Good” for example in B, and those famous riffs are pretty much all straight piano stuff AFAIC just taken over to guitar.
“shredding” is when you’re good and can rip up a solo like your fingers are robotic and your mind is a melody machine on overdrive.
“wanking” is what the OP describes. And yeah, I think that Jack Black nailed it in that scene in School Of Rock.
I’ve always thought of ‘noodling’ being nothing like wanking. Compare what the McFly character does in the BTTF scene, to a guitar solo by someone like Frank Zappa, who considered himself a ‘noodler’. Big difference, in my opinion.
Fretty Sanchez
All good and true. I play JBG in A simply because of the pull offs and hammer-ons on guitar, so know that Johnson was an influence in the choice of a non-guitar key.
Band seeking drummer. Must be able to grimace musically.
Thanks for the suggestions! 
I think I was going for terms to describe what Marty was doing specifically, as well as terms to describe whatever real life musician he was parodying was doing.
I think it was a movie conscious shred, in the Satriani style… very composed and technical. Fox’s solo was a deliberate and cynical play on the Chuck berry, and coming sixties style, a bit before its time in the 50’s. It was an indulgent overeach of “modern times” and historical nostalgia as imposed by the director.
Maybe he was supposed to start the Geetar Revolution by exposing up and commers to the sound… typical time travel insertion and influence on the future. Violating the prime directive. Everything about back to the future seems to be about making sure he was born and subsequently born into the “right time”. Past quantum and temporal movements having larger impacts in the future.
I was serious about the skronk.