To gradually weaken or destroy the strength or vitality of something or someone. The long hours of work began to sap his energy.
To undermine or weaken a structure or foundation, often by removing support. The constant erosion sapped the foundation of the old bridge.
Military. To dig a sap or approach trench toward an enemy fortification. Throughout the night the engineers sapped silently toward the fortress walls.
To strike or knock unconscious with a sap (blackjack). The guard was sapped from behind before he could raise the alarm.