I have also experienced the wrath of an elderly tyrant on a bus. I was in the wrong in the first place, but her words and tone were very much uncalled for. I had had very little sleep, and was daydreaming. She walked over, and said "Are you disabled, are you?". Reading it as text, this may not sound very offensive, but I'm sure that with a bit of imagination you can come up with an appropriately rude tone of voice to make it sound somewhat comparable to how it really was. It was very tempting to say "Yes, I have a mental disability, as diagnosed by qualified experts, and recognised by the government, so suck it!", and I assure you that this is what ran through my mind even as I held my tongue and relinquished my seat. In hindsight, I'm glad I didn't. As well as being a matter of chivalrous principle, one never knows exactly what another has been through, and whilst her actions were uncalled for by the principles of chivalry themselves, it's near to the very definition of chivalry to give one and all the benefit of the doubt, and treat them accordingly.
I suppose what I just said really sums up just what it takes to be chivalrous: "Give one and all the benefit of the doubt, and treat them accordingly." This, in itself, costs nothing but a level of self-control that society demands of you in other ways anyway, so you might as well do it, right?
EDIT: Corrected spelling mistake