Select the word that is incorrectly spelt.

Correct Answer: Meretricous

Explanation:
Let us analyze the spelling and meaning of each word given in the options:

1. Meretricous: This word is incorrectly spelt. The correct spelling is Meretricious (it requires an 'i' before the '-ous' suffix).
Meaning: Apparently attractive, but having in reality no value or integrity.

2. Pugnacious: Correctly spelt.
Meaning: Eager or quick to argue, quarrel, or fight.

3. Perspicacious: Correctly spelt.
Meaning: Having a ready insight into and understanding of things; perceptive.

4. Magnanimous: Correctly spelt.
Meaning: Generous or forgiving, especially toward a rival or less powerful person.

Therefore, the incorrectly spelt word is Meretricous.