The presence of other Old Japanese indefinites beginning with い (i) suggests a possible shared root of い (i, possibly “what” or “which”), as in 如何(ikaga, “how”), 何処(izuko, “where”), 幾ら(ikura, “how many, how much”).
Likely cognate with eastern Old Japanese word idu (modern idzu or izu, “where”), still used in modern Japanese as part of the word いずれ(izure, “which”).