'Mercilessness' definitions:

Definition of 'mercilessness'

noun

Feelings of extreme heartlessness [syn: cruelty, mercilessness, pitilessness, ruthlessness]

noun

Inhumaneness evidenced by an unwillingness to be kind or forgiving [syn: mercilessness, unmercifulness] [ant: mercifulness, mercy]

From: GCIDE

Merciless \Mer"ci*less\, a. Destitute of mercy; cruel; unsparing; -- said of animate beings, and also, figuratively, of things; as, a merciless tyrant; merciless waves. [1913 Webster]
The foe is merciless, and will not pity. --Shak. [1913 Webster]
Syn: Cruel; unmerciful; remorseless; ruthless; pitiless; barbarous; savage. [1913 Webster] -- {Mer"ci*less*ly}, adv. -- {Mer"ci*less*ness}, n. [1913 Webster]