From The Jargon File (version 4.4.7, 29 Dec 2003): drain v. [IBM] Syn. for flush (sense 2). Has a connotation of finality about it; one speaks of draining a device before taking it offline.
drain v.
[IBM] Syn. for flush (sense 2). Has a connotation of finality about it; one speaks of draining a device before taking it offline.