This word leannoth seems to point to some kind of instrument unknown ( Psalms 88 , title). The whole phrase has by others been rendered, "On the sickness of affliction: a lesson;" or, "Concerning afflictive sickness: a didactic psalm."
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Bibliography InformationEaston, Matthew George. "Entry for Mahalath Leannoth Maschil". "Easton's Bible Dictionary". .