In one sentence…
What is the meaning (and for which parts), the implications, and, if possible, the etymology of these three terms?
In more words…
When I hear this bike has more aggressive geometry than that one, I gather that the position of the cyclist will be lower, and hence more aerodynamic—in other words, faster, but less comfortable.
But I'm not too sure what to conclude when I hear this bike has a slacker head tube angle. I understand that the head tube will start to look like a Harley Davidson motorcycle. I also vaguely gather that it will be easier to hit small obstacles at speed, but my reaction remains "yes, and?".
Another term that the pundits throw around in online fora is this bike has more relaxed geometry, which can be an umbrella term for a longer wheelbase, but then, oddly, the cyclist's position also becomes more aggressive, which may be distinct from talking about the bicycle's geometry being aggressive, even if both imply, say, a longer wheelbase.
Elsewhere also the implications will vary wildly. A slacker seat tube angle—at an extreme such as that on those Townies that enable a flat foot on the ground while seated—means a less aggressive bike, rather than the aggressive one implied by a slacker head tube angle.
Can you provide the 101 of this jargon trio, sufficient to read cycling magazine reviews?
Also, if you've figured out why a clothing or attitude term (slack-er) rather than a geometry term (acute/obtuse), and a temperament term (relaxed) came to be used in cycling, do share.
- An excellent introductory exposition appears here.