That in an ultrasound shows a smaller than expected reflex, or to the reference mean, because it returns little ultrasound signal. See ecoic, ecogenic, hyperecoic .
It means that it produces less echo or bounce of waves. In Medicine are nodules that look a little darker in an ultrasound image. It is because they have low ecogenicity i.e. the ability to reflect ultrasound waves (brightness intensity of an ultrasound image).