1) Slang for studying hard 2) Undermining 
Sapping is a term used in siege operations. Any trench excavated under defensive musket or artillery fire that was intended to advance a besieging army`s position in relation to the works of an attacked fortification was referred to as a sap. Saps of approach were excavated by brigades of trained soldiers, often called sappers, because they dug t....
• (p. pr. & vb. n.) of Sap
(from the article `valley`) Sapping is a process of hillslope or scarp recession by the undermining of an overlying resistant material in the form of weathering or water flow ...
Sapping is slang for studying hard.
A process of erosion where water leaks to the surface through the pores of rocks; as the water flows away, it slowly removes material to form valleys and channel networks.