The story
From tool + tip, originally used with toolbars to provide a tip or hint as to what function each graphical icon represented. The term was introduced as ToolTip by Microsoft in the 1990s.
Etymology from Wiktionary, CC BY-SA 4.0.
tooltip
tool·tipToday it means: An element of a graphical user interface in the form of a box of text that appears when a cursor is made to hover over an item; normally used to explain the function of the item. Full definition →