“Whelm” means to “engulf, submerge, or bury.” I can understand “overwhelming” but “underwhelming” makes no sense. If the water is just at your height, you are submerged; any amount of water beyond it would be “overwhelming,” but any amount less would not be “whelming” at all as you are not “submerged” yet.
“Overachieving” makes sense in the same way “overwhelming” does, but again, “underachieving” doesn’t make much sense because achieving is also binary. You either achieved it or didn’t (submerged or not). Anything above it is fine but going below it contradicts the very definition of the word.
You can “overstate” or “understate” something because stating has no objective point at which it qualifies as or ceases to be stating. It’s subjective. The same structure holds for “overgeneralize,” “overpriced,” and “overact.”
“Overabundance” is silly in that “abundance” already implies excess; there is no need to add more excess to excess. “Irregardless” is silly also in that it means the same thing as “regardless,” and adding “ir” doesn’t even overemphasize it.
The most ironic case is “overthink.” Just as in “understate,” thinking has no objective level that is deemed appropriate. Yet the word “underthink” is not in Webster, Cambridge Dictionary, or Dictionary.com. The cases of not thinking enough are rampant. Unlike the consequences of overthinking, underthinking is often detrimental. Clearly, the world is biased against overthinking and tacitly condones underthinking. If you are not going to use the word “underthink,” it’s not fair to use the word “overthink.”
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