Eye-Opening Entomology

...the complexities of “adult” and “adultery.” How can such similar words have such different meanings?

Remarkably, the answer is that the words don’t share a common ancestor. “Adult” comes from the Latin verb adolescere, “to grow up, mature.” Students of Latin will understand what we mean when we say that adultus is the pluperfect of adolescere. Adultery, on the other hand, derives from a French word, avoutre, which in turn evolved from a distinct Latin verb, adulterare, “to corrupt.” The verb adulterate, “to debase or make impure by adding inferior materials or elements,” stems from the same source.

The sense of “adult” that means pornographic emerged as a kind of reverse assumption that adult and adultery have more direct links than they do.

-from Dictionary.com