early 15c., earlier asile (late 14c.), "place of refuge, sanctuary," from Latin asylum "sanctuary," from Greek asylon "refuge, fenced territory," noun use of neuter of asylos "inviolable, safe from violence," especially of persons seeking protection, from a- "without" (see a- (3)) + syle "right of seizure," which is of unknown etymology.
Literally, "an inviolable place." Formerly a place where criminals and debtors sought shelter from justice and from which they could not be taken without sacrilege. General sense of "safe or secure place" is from 1640s; abstract sense "inviolable shelter, protection from pursuit or arrest" is from 1712. Meaning "benevolent institution to shelter some class of persons suffering social, mental, or bodily defects" is from 1773, originally of female orphans.