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Webster 1828 Edition

adv.

With compassion or pity; tenderly; mildly.

mercifully ‎(comparative more mercifully, superlative most mercifully)

In a merciful manner.
thankfully
- 2014, Steve Rose, "Dawn of the Planet of the Apes: a primate scream - first look review", The Guardian, 1 July 2014:
  The whole Planet of the Apes set-up has been ripe for metaphor – from slavery and Afro-American revolution to European conquest of the Americas, even the war on terror. But mercifully, there's no big subtext being troweled on here.