I'd guess it's the other way around. 'Autonomous' is an unusual enough word that you remember what it means (especially once you understand the 'auto' prefix),

I'd guess it's the other way around. 'Autonomous' is an unusual enough word that you remember what it means (especially once you understand the 'auto' prefix), or look it up because it's strange. 'Agent' is so familiar though that I can imagine lots of people don't attach its literal meaning to it in the sense of 'having agency'. Popular uses are things like 'secret agent', 'cleaning agent'... "probably means like, 'person', or 'thing' or something".