[zahlman]

Unclear, but we do have https://www.catb.org/jargon/html/X/XXX.html

> Some hackers liken ‘XXX’ to the notional heavy-porn movie rating.

This seems plausible given the older culture ("this is metaphorically dirty, and therefore like porn", insert puerile snickering) and I can recall old jokes about "searching for" these markings. But I think it's also just about it visually standing out - the X character filling the terminal display cell with sharp lines.

[o11c]

Hm, this made me wonder about the use of XXX as a label for alcohol bottles in cartoons.

It turns out it refers to "moonshine that has been distilled 3 times, reaching very high alcohol content".