Adjective

(en adjective)

hideous or frightful

* Shakespeare

So bad a death argues a monstrous life.

enormously large

a monstrous height

a monstrous ox

freakish or grotesque

* John Locke

a monstrous birth

* Jeremy Taylor

He, therefore, that refuses to do good to them whom he is bound to love is unnatural and monstrous in his affections.

of, or relating to a mythical monster; full of monsters

* Milton

Where thou, perhaps, under the whelming tide / Visitest the bottom of the monstrous world.

(obsolete) marvellous; strange

Adjective

(en adjective)

Harmful, malevolent, injurious.

* {{quote-book, year=1922, author=(Ben Travers), title=(A Cuckoo in the Nest)

, chapter=1 citation , passage=“[…] the awfully hearty sort of Christmas cards that people do send to other people that they don't know at all well. You know. The kind that have mottoes

(medicine) Tending to produce death; threatening a fatal issue.

malignant diphtheria

a malignant tumor