From Old English hlǣdder, from PIE *klei- 'to lean' — a ladder is literally something you lean against a wall.
A structure of horizontal bars fixed between two upright lengths of wood, metal, or rope, used for climbing.
From Old English hlǣdder 'ladder, steps,' from Proto-Germanic *hlaidrī, from PIE *klei- 'to lean.' The ladder was named as something you lean against a wall. The initial h- was lost by Middle English. Key roots: *klei- (Proto-Indo-European: "to lean, incline").