厳しい

Originally from regular classical adjective 厳し (kibishi, “tight, close, without gaps”), first cited to roughly 830.[1]

This earlier form later shifted conjugation patterns, from the so-called "-ku adjective" to the "-shiku adjective" pattern. The two conjugation types appear to have been in broadly free variation, with both forms developing the strict; severe senses in parallel.[1] Historically, while there was some overlap, there does appear to be a sense distinction in Classical Japanese texts, wherein the older "-ku adjective" form was used more commonly to express the more objective qualities of "tight, close", while the "-shiku adjective" was used more commonly to express the more subjective qualities of "severe; harsh".[1]

The "-shiku adjective" pattern is first cited from the late 900s.[1]

• (Tokyo) きびし​い [kìbíshíꜜì] (Nakadaka – [3])[2]
• IPA^(key): [kʲibʲiɕii]

厳(きび)しい • (kibishii) -i (adverbial 厳(きび)しく (kibishiku))

1. [from late 900s] stern, grave, forbidding; dignified, stately
2. [from early 1000s] strict, unforgiving
3. [from 1116] tight, close, without gaps
4. [from 1179] harsh, tough, extreme, severe, hard to withstand (said of natural phenomena)

   厳(きび)しい山(やま)、厳(きび)しい猛暑(もうしょ)、厳(きび)しい環境(かんきょう)

   kibishii yama, kibishii mōsho, kibishii kankyō

   a tough (steep) mountain, a severe heat wave, a harsh environment

5. [from 1710] extreme, excessive, terrible (said of an action)

   あの打撲(だぼく)は厳(きび)しいよ！

   Ano daboku wa kibishii yo!

   That's a terrible blow!

6. [from late 1700s?] dire, difficult, tight (said of a situation)

   読(よ)むのが厳(きび)しい

   yomu no ga kibishii

   difficult to read

• (harsh): きつい (kitsui)
• (strict): 厳格(げんかく)な (genkaku na), 堅(かた)い (katai), (Kagoshima) いね (ine)
• (terrible): 激(はげ)しい (hageshii)

• 厳(きび)し (kibishi, classical)
• きぶい (kibui)
• きびきび (kibikibi, adverb)

From the 連体形 (rentaikei, “attributive form”) of Early Middle Japanese adjective 厳し (itsukushi), with the medial /-k-/ falling out.

• IPA^(key): [it͡sɨᵝkɯ̟ᵝɕii]

厳(いつく)しい • (itsukushii) -i (adverbial 厳(いつく)しく (itsukushiku))

1. [from 759] (rare, archaic) wondrous, miraculous, splendid, sublime
2. [from early 1000s] (rare, archaic) dignified, solemn, stern, noble, refined
3. [from 1420] (rare, archaic) beautiful, pretty
   • c. 14-16th century: Kazashi no Himegimi

     姫君一人おはします。[...] 御かたちを見るに、髪のかかり、眉、口つき、いつくしくて [...]

• 慈(いつく)しむ (itsukushimu)

• Shōgaku Tosho (1988) 国語大辞典（新装版） [Unabridged Dictionary of Japanese (Revised Edition)] (in Japanese), Tōkyō: Shogakukan, →ISBN
• 2002, Ineko Kondō; Fumi Takano; Mary E Althaus; et. al., Shogakukan Progressive Japanese-English Dictionary, Third Edition, Tokyo: Shōgakukan, →ISBN.