different between unrelenting vs unforgiving

English

un- +? relenting

unrelenting (comparative more unrelenting, superlative most unrelenting)

Not relenting; not yielding in strength, severity, or determination.
Synonyms: unyielding, brutal, hard, harsh, stern, cruel
Having no pity; not being or becoming lenient, mild, gentle, or merciful

unrelenting in The Century Dictionary, New York, N.Y.: The Century Co., 1911.
unrelenting in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.

un- +? forgiving

unforgiving (comparative more unforgiving, superlative most unforgiving)

Unwilling or unable to forgive or show mercy.
The electorate was in an unforgiving mood.
Having no allowance for weakness.
The unforgiving Arctic climate makes short work of the unprepared.

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