Is it ensure, insure, or assure?
Old English dreahnian "to drain, strain out," from Proto-Germanic *dreug-, source of drought, dry, giving the English word originally a sense of "make dry." Figurative meaning of "exhaust" is attested from 1650s. The word is not found in surviving texts between late Old English and the 1500s. Related: Drained; draining.
1550s, from drain (v.).
A device, such as a tube, inserted into the opening of a wound or into a body or dental cavity to facilitate discharge of fluid or purulent material. v. drained, drain·ing, drains
To draw off a liquid gradually as it forms.
(IBM) To allow a system to complete the processing of its current work before the system becomes unavailable. E.g. draining a device before taking it off-line or telling a web server in a server farm not to accept any new requests but to finish processing any requests it has already accepted.