The term pocket veto refers to an indirect veto of legislation by the president who refrains from approving and signing or officially vetoing the legislation within the approval period (10 days for the U.S. President) before Congress adjourns the legislative session. It is a strategic maneuver used to take indirect action through inaction in order to kill legislation without openly vetoing the legislation.
President Anderson did not want to openly veto the bill as the public would not understand why he would kill a seemingly good bill so he chose to run out the clock knowing that Congress would adjourn in 3 days and the bill would die without him having to publicly veto it.
U.S. Senate Definition
The Constitution grants the president 10 days to review a measure passed by the Congress. If the president has not signed the bill after 10 days, it becomes law without his signature. However, if Congress adjourns during the 10-day period, the bill does not become law. Summary of Bills Vetoed, 1789-present