Simple has a rich etymology and has kept most of its meanings throughout the centuries. It began life in the early 1200s as meaning humble or ignorant:

They were simple, ignorant, harmless children of Nature; with no knowledge of soul, mind, God or heaven.

By the late 1200s, simple also meant low-ranking:

This language, in spite of the complex influences which went to the making of the nationality of France, was of a simple origin.

About 1300, it had add pure to its list of meanings, and in the early 1400s, it also meant made of just one thing, the opposite of composite. By 1550, simple could also be used to mean not complicated.

Sometime during all this, simple was also used as a noun to mean a plant or an herb used for medicine. To simple was to collect such a plant, and a simplist was someone who did the collecting. If a given plant had simplistic virtues, it had the virtues of a simple. If your cousin had simplistic talents, she was good at making use of simples.

The medicinal simple has long since disappeared from everyday English, but simplistic has survived. By 1881, it had come into its modern meaning of trying to explain something complicated as being simpler than it is; that is, oversimplifying:

Joe Ollinger offers simplistic answers to the complex issues we face.

What simplistic does not mean, however, is simple or easy:

The game offers simplistic controls for the entire family to enjoy.

Don't be fooled by salesmen pitching simplistic products. Unless he's selling medieval medicine, he's probably too simple to know what he is selling.