"Insidious"

When:

Lookups started climbing in early March, and it eventually became the most looked-up word for the month.

Why:

The movie Insidious opens in theaters on April 1. Directed by the creator of the ultra gory Saw franchise, the new film has generated enormous pre-release curiosity.

Insidious means "causing harm in a way that is gradual or not easily noticed" or "harmful but enticing" – promising definitions for fans of horror flicks.

It comes from the Latin word that means "ambush."