From BiblePortal Wikipedia

King James Dictionary [1]

Mer'Ciful, a. from mercy. Having or exercising mercy compassionate tender disposed to pity offenders and to forgive their offenses unwilling to punish for injuries applied appropriately to the Supreme being.

The Lord passed before him and proclaimed, the Lord, the Lord God, merciful and gracious, long-suffering and abundant in goodness and truth.  Exodus 34

1. Compassionate tender unwilling to give pain not cruel. A merciful man will be merciful to his beast.

Webster's Dictionary [2]

(1): ( a.) Full of mercy; having or exercising mercy; disposed to pity and spare offenders; unwilling to punish.

(2): ( a.) Unwilling to give pain; compassionate.