If a substance such as acid or rust eats into something, it destroys or damages its surface. Ulcers occur when the stomach's natural acids eat into the lining of the stomach
gnaw into; make resentful or angry; "The unjustice rankled her"; "his resentment festered"
If something eats into your time or your resources, it uses them, when they should be used for other things. Responsibilities at home and work eat into his time
take a chunk out of, erode, eat away (especially referring to financial or other savings)
gnaw into; make resentful or angry; "The unjustice rankled her"; "his resentment festered