What part of speech is unthaws?

Unthaws can be categorized as a verb.

Parts of speech

1. unthaws is a verb, present, 3rd person singular of unthaw (infinitive).

Verb
unthaw - 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"