Other forms: overstimulated; overstimulating; overstimulates
Overstimulate means to overwhelm someone, or make something overactive. Feeling angry, frustrated, or upset in a noisy or brightly lit space might suggest that you're easy to overstimulate.
Caffeine might overstimulate your brain and make it difficult to sleep, or loud music which is fun at first might eventually make you feel overwhelmed, or overstimulated. Stimulate means "to provide stimulus or input." Overstimulate means "to provide an overwhelming amount of input."