From mortal + -ly.
mortally (comparative more mortally, superlative most mortally)
- Fatally; in such a way as to cause death. [from 14th c.]
- (obsolete) As a mortal. [16th-17th c.]
- 1596, Edmund Spenser, The Faerie Queene, V.1:
- And all the depth of rightfull doome was taught / By faire Astræa with great industrie, / Whilest here on earth she lived mortallie […].