Per Padolsey, "Truthy: Something which evaluates to TRUE. Falsey: Something which evaluates to FALSE." But like many things in programming, it requires a bit of exploration to understand that falsey/falsy is true (or accurately false) in five cases: undefined, null, NaN, 0, "" (empty string), and false. These two articles can give you a good overview:
Snrky: When you reply to my emails, why is my quoted text always preceded by a !! ?
Title: Because you're exciting? (and Boolean seemed extraneous)