nidan verb (lower class) with 'tsu' ending (archaic), intransitive verb
Archaic intransitive verb (Nidan, lower class, -tsu ending). Modern equivalent is 凍てる (いてる) or 凍る (こおる). Found only in classical texts.
See also: 凍てる
古語では「水が凍つ」のように用いた。
In classical Japanese, it was used like 'the water freezes'.
Classical Japanese verb; the exact derivation is uncertain, but it is related to modern 凍てる (いてる).