In a kind, compassionate way that shows mercy, or conveniently/fortunately reducing suffering. Mercifully, the pain medication worked quickly.
From mercy (from Old French mercerie, from Latin merces meaning wages/reward) plus -ful plus -ly. The word evolved from the sense of showing compassion to the sense of fortunate relief from hardship.
Mercifully is one of those words that works in two ways—you can be mercifully treated by someone kind, or something can be mercifully brief (lucky). This double meaning reflects how medieval people saw mercy: both as active compassion and as divine grace that intervenes to help.
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