Hideous or frightful. / Enormously large. / Freakish or grotesque. / Of, or relating to a mythical monster; full of monsters. / (obsolete) Marvellous; exceedingly strange; fantastical.

(mathematics) The unexpected connection between the Monster group and modular functions, now known to relate to a certain conformal field theory having the Monster group as symmetries.

In a monstrous manner.

plural of monstrousness

(uncountable) The state or condition of being monstrous. / (countable) Something monstrous.

superlative form of monstrous: most monstrous