Of personal actions, character, feelings, or utterances: (a) fierce, furious, cruel (obsolete or archaic); (b) (in modern use) stern, unrelenting…

Of a person, his qualities, etc.. Hard and cold as steel, unimpressionable, inflexible, obdurate.

Unbendable, inflexible.

Of persons, their actions or attributes.

Not giving way to feelings of kindness, compassion, or mercy; unyieldingly harsh, stern, or strict. Applied to persons, personal attributes, etc.

= relentless, adj.

Harsh in impact or effect; characterized by sternness or severity.

That cannot be appeased; irreconcileable; inexorable: of persons, feelings, etc.

That cannot be made to relent or give way; implacable.

(un-, prefix1 affix 2.)

(un-, prefix1 affix 4.)

Rigid, strict; solemn, formal.

(un-, prefix1 affix 4.)

(un-, prefix1 affix 4.)

Resolute in dealing with opposition; vigorously uncompromising; severe; esp. in to get tough (cf. get, v. compounds c). colloquial (originally U.S.).