In one of the Bleach anime ending songs, "Hanabi" has the following line:
繋ぎゆく この想い 愛しき 君
"itoshiki" seems to come from "itoshii", but how does this -ki form of i-adjective work? I found a few other adjectives that have -ki forms, e.g. 幼き{おさなき}, 素晴らしき{すばらしき}, 古き{ふるき}, 良き{よき} etc but that's about it.
Is this form productive? Is it selective, i.e. only some i-adjectives can have this form? Or has it become archaic, and thus only limited to those that survived into present Japanese?