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Webster 1913 Edition

Adj.

Full of threats; having a menacing appearance.

Spenser.

–

Threat′ful-ly

adv.

Adj.

thret'ful. Full of threats; having a menacing appearance; minacious.

threatful ‎(comparative more threatful, superlative most threatful)

Threatening.
- 1590, Edmund Spenser, The Faerie Queene, II.xii:
  Then he with all his puissance doth striue / To strike his oares, and mightily doth driue / The hollow vessell through the threatfull waue [...].