This word, a bit of a linguistic curio, always travels in tandem, like Thomson and Thompson or Tweedledee and Tweedledum: curiouser and curiouser. Why? Because it’s always used in reference to a specific literary usage: Alice, in Alice in Wonderland by Lewis Carroll (Charles Dodgson). Why does she say it? “She was so much surprised, that for the moment she quite forgot how to speak good English.” Why was she so surprised? She was opening out “like the largest telescope that ever was.” It was an unanticipated effect of eating a very small cake.
How very odd. But it’s just as odd to see the comparative suffix er on such a long word. It’s as though a short word had telescoped out (that takes the cake). Or perhaps her English was getting spuriouser and spuriouser.
But actually, although we now as a rule restrict use of this er to adjectives of one or two syllables, and without suffixes other than y or ly, the er is human and humans have er-ed at greater length without being disingenuous: beautifuller used to be acceptable, and eminenter. But the acceptable stems are now shorter and otherwise restricteder. Just as well, with a half dozen er suffixes to choose from (there’s the one on singer, the one on flicker and flutter, the one on disclaimer – from an infinitive – and a couple of others).
As to the stem in this case, curious, it is a rather curious word, and not just because it can be applied to the object that inspires curiosity; it comes from Latin curiosus “careful, assiduous, inquisitive” and has meant quite a few things in its time: painstaking, fastidious, clever, skillful, naughty… And curious goes with an assortment of other words, notably George (while curiosity for its part killed the cat), whereas curiouser goes with another of itself… and Alice.