Definitions for unthaw
dissolve, thaw, unfreeze, unthaw, dethaw, melt(verb)
become or cause to become soft or liquid
"The sun melted the ice"; "the ice thawed"; "the ice cream melted"; "The heat melted the wax"; "The giant iceberg dissolved over the years during the global warming phase"; "dethaw the meat"
To not thaw; to (re-)freeze or remain frozen.
To thaw out, to unfreeze; to become soft (of something which had been frozen).
Origin: The senses, opposites in meaning, are from un-#Etymology 2 + thaw and un-#Etymology 2 + thaw, respectively. The sense "not thaw, remain frozen" is from the late 1500s.