いずこ

⟨iduku⟩ → /id͡zuku/ → /id͡zuko/ → /izuko/

Shift from Old Japanese-derived いずく (izuku) of the same meaning.

By analogy with locative pronouns ここ (koko) and そこ (soko), both of which end in -ko, the final -u of izuku became -o. Furthermore, the final the -ku and -ko of both aforementioned pronouns ultimately share cognancy with Old Japanese 処 (-ko₂), which would have been pronounced as /kə/ at that time. (Can this⁽⁺⁾ etymology be sourced?)

• (Tokyo) いずこ (Atamadaka – )^[1]
• (Tokyo) いずこ (Heiban – )^[1]
• IPA^(key):

いずこ • (izuko) ^{←いづこ (iduko)?}

1. (archaic) indefinite interrogative pronoun: where, whence, whither, which
   • c. 935, Ki no Tsurayuki, Tosa Nikki (Jōhei 5, twenty-ninth day of the first month)^[2]

     おもしろき所(ところ)に船(ふね)を寄(よ)せて「こゝや何(いづ)処(こ)」と問(と)ひければ「土(と)佐(さ)の泊(とまり)」と言(い)ひけり。

     Omoshiroki tokoro ni fune o yosete, “koko ya izuko,” to toikereba, “Tosa-no-tomari” to iikeri.

     As the boat arrived at some fascinating place I asked, “Whither here?” He said, “Tosa Harbor”.

   • 1998, “Yuki Shinshin [Snow Falling Heavily]”, Miyuki Ishimoto (lyrics), Satoshi Hinokibara (music), performed by Ayako Fuji:

     肌(はだ)を寄(よ)せた愛(あい)を失(な)くし　胸(むね)に淋(さび)しさ突(つ)き刺(さ)さる
     しんしんしん雪(ゆき)が舞(ま)う舞(ま)う　しんしんしんしん夜(よ)明(あ)けはいづこ

     hada o yoseta ai o nakushi mune ni sabishisa tsukisasaru
     shin-shin-shin yuki ga maumau shinshin-shinshin yoake wa izuko

     Coming to my skin, love has been lost―my heart is piercing with loneliness!
     Falling heavily, the snow is dancing, dancing―it's falling very heavily, where is the dawn?

2. (archaic, after a particle) indefinite demonstrative pronoun: anywhere, everywhere, somewhere
   • 905–914, Kokin Wakashū (book 3, poem 166; also Hyakunin Isshu, poem 36)

     夏(なつ)の夜(よ)はまだ宵(よひ)ながらあけぬるを雲(くも)のいづこに月(つき)やどるらむ

     natsu no yo wa mada yoi nagara akenuru o kumo no izuko ni tsuki yadoruran

     The summer night dims as evening hours have not yet passed―somewhere in the clouds the moon takes her refuge.