As far as I can tell, both mean foolish. Checking a dictionary, 愚かしい is defined as 愚かである。ばかげている。which does not help very much. Is there truly no real difference?
2 Answers
愚か（だ） describes the quality of "absent of wiseness", while 愚かしい, as with many i-adjectives that end in -しい, indicates a kind of mental perception such as sense, emotion or impression. In this sense, 愚かしい is actually telling "felt unwise" or "as if the unwise do", which is interestingly close to English approach to say foolish that is fool + -ish.
Thus you can say these two has slightly different connotation when both are usable:
戦争の愚かさ
(emphasizes how nonsense of (people to do) wars)戦争の愚かしさ
(emphasizes how (people doing) wars look irrational)
And it's not totally impossible to find some examples only either one sounds valid:
○ 愚かな人間には理解できない foolish humans will not understand
× 愚かしい人間には理解できない
The latter is unacceptable because 愚かしい does not imply inherent, permanent character of foolishness.
answered Jun 29, 2018 at 8:53
The two words are similar in meaning and usage. However, two significant differences stand out.
愚{おろ}かしい is much less commonly used than 愚{おろ}かな.
愚かしい is virtually never used to refer to people, whereas 愚かな certainly can.
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愚かしい is virtually never used to refer to people-> But... this dictionary gives an example sentence 「何と愚かしい人間だ」... あと「愚かしい男」「愚かしい自分」「愚かしいやつ」などをググるといくらかヒットしますが– chocolate ♦Feb 11, 2017 at 4:04 -
Google search with quotes for exact matches: 愚かな人 has over 400 times as many results as 愚かしい人. 愚かな男 results in 320 times as many hits. 人間 results in 80 times as many hits. 者 yields 40 times as many results. I consider that to be rare usage.– Alex KFeb 11, 2017 at 7:59
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(1) Google search doesn't quantify precisely ... so don't depend on that.– virmaiorJun 29, 2018 at 16:08
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similar in meaning and usagedoesn't make any sense when one ismuch less commonly used.– virmaiorJun 29, 2018 at 16:11
[使い分け] 「愚かしい」は、ばかげているさまの意でも用いられる。