Arabic hijr حجرwhich, in its primitive sense, means “interdiction or prevention.” In the language of the law it signifies an interdiction of action with respect to a particular person; the causes of inhibition being three: infancy, insanity, and servitude.
The acts of an infant, i.e. one under puberty, and unlawful, unless sanctioned by his guardian. The acts of a lunatic who has no lucid intervals are not at all lawful; and so are those of a male or female slave. (Hidayah, iii. p. 468.)
Based on Hughes, Dictionary of Islam
INITIAL LETTERS of the Qur’an