1910年，医学拉丁语，意为“最高级别的弱智人士之一”，源自希腊语（雅典方言）mōron，是mōros的中性形式，意为“愚蠢的、迟钝的、懒散的、愚笨的”，这个词的起源尚不明确。它与梵语murah（意为“愚笨的”）的联系（参见moratorium）也存在疑问。拉丁语中的morus（意为“愚笨的”）是从希腊语借来的。
该词被美国弱智研究协会采纳，技术性定义为“智力年龄在8到12岁之间的成年人”；自1922年起被用作侮辱性词汇，随后在技术上被弃用。林奈曾引入morisis（意为“愚笨症”）。
The feeble-minded may be divided into: (1) Those who are totally arrested before the age of three so that they show the attainment of a two-year-old child or less; these are the idiots. (2) Those so retarded that they become permanently arrested between the ages of three and seven; these are imbeciles. (3) Those so retarded that they become arrested between the ages of seven and twelve; these were formerly called feeble-minded, the same term that is applied to the whole group. We are now proposing to call them morons, this word being the Greek for "fool." The English word "fool" as formerly used describes exactly this grade of child—one who is deficient in judgment or sense. [Henry H. Goddard, in "Journal of Proceedings and Addresses" of the National Education Association of the United States, July 1910]
弱智人士可以分为以下几类：（1）在三岁之前智力发展完全停滞，表现出两岁以下儿童的水平，这类人被称为白痴；（2）在三到七岁之间智力发展严重滞后，最终智力停滞不前，这类人被称为弱智者；（3）在七到十二岁之间智力发展滞后，最终智力停滞，这类人曾被称为弱智者，与整个群体的称谓相同。我们现在提议称他们为愚人，这个词在希腊语中意为“愚蠢的人”。英语中的“愚人”一词早期正好描述了这种儿童——缺乏判断力或常识的儿童。[亨利·H·戈达德，发表于1910年7月《美国国家教育协会会议记录与演讲集》]
