unmerciful

adjective

1. merciless; relentless; severe; cruel; pitiless.
2. unsparingly great, extreme, or excessive, as amounts: to talk for an unmerciful length of time.

adjective

1. showing no mercy; relentless
2. extreme or excessive

adj.late 15c., from un- (1) “not” + merciful. Related: Unmercifully.