Why does 不甲斐{ふがい}ない mean "worthless" while 甲斐{かい} means "worth"? JMdict: > * {}: disappointing; weak-minded; spiritless; cowardly; worthless; pusillanimous; feckless; faint-hearted; shiftless; tame; timid > * {}: effect; result; worth; use; avail > The word has both and around , and to my understanding the prefix - is used to negate words, just like . By this logic, the meaning of the word should be "not-worthless" instead of anything negative. Of course language doesn't work that simply and there must be even bigger "illogicalities" out there that I've just gotten used to, but maybe there is a reason for this one. * * * **Edit** : there is also the word [[]{}]( (worthless; pointless; useless​) as pointed out by @naruto in comments. Seems like this one works as I would expect.

was originally . is an uncommon kanji meaning "gut." According to this article, was much more commonly used by novelists in the Meiji and Taisho periods. According to this chiebukuro question, the first edition of , one of the most authoritative Japanese dictionaries, had only as the kanji for .

For some reason came into use and somehow overtook during the Showa period. Many sites believe this was basically a misuse. Every so often controversial usages of words may become acceptable gradually over time...

BTW there are similar word pairs where doesn't appear to change the meaning, for example /, /, /. See the following article for details. I initially thought was also an example of this, but the case of seems to be another story.

* Q&A