I’ll grant that perhaps this was the case with your drill sergeant, but a spiderweb on the elbow (other joints, shoulder is common too) has a general meaning of entrapment- as in, the wearer has felt bonded/enslaved/caged, be it physically a la prison time (web lines indicating years served) or a mental health facility, or more meta, i.e identity issues, or general life circumstances like say growing up with an abusive caretaker or being addicted to drugs.
A friend of mine has a spider web on her elbow marking the time it took her to transition. Mine reflects my apathetic early adulthood, couch surfing, squatting, burning out.
Historically speaking, the spider web originally implied prison time, but not just for murder, really for anything. I suppose it was just another quick way to identify yourself to other cons, a business card of sorts…
But yeah, like many a symbol it has derivated countless times since origination to the point where any one persons’ spiderweb tattoo means nothing to you, and everything to them.
Also, that SPLC link, while getting some of the easy, obvious ones like rahowa or 1488, wildly oversimplifies or outright miscategorizes many of the other examples, particularly concerning skinhead culture which is not inherently racist.
This is a better link, anyway, though I’d hesitate to call it as authoritative as the title suggests.
https://amp.www.complex.com/life/2017/01/dictionary-of-white-supermacist-slang-symbols/
