Definition from Wiktionary, the free dictionary
Etymology 1 
- (transitive) to cook something in simmering water
- To become soft or muddy.
- Chalky and clay lands […] chap in summer, and poach in winter. — Mortimer.
- To make soft or muddy.
- Cattle coming to drink had punched and poached the river bank into a mess of mud.
cook in simmering water
Etymology 2 
- (transitive, intransitive) to take game or fish illegally while trespassing on someone's property
- (transitive, intransitive) to take anything illegally or unfairly
- (transitive, intransitive) to cause an employee or customer to switch from a competing company to your own company
Derived terms 
to take game or fish illegally
to take illegally or unfairly
to cause an employee or customer to switch