Plassey is the anglicized form of the word Palashi. The area is named due to the abundance of the Palash trees. 

Plassey was the site of a British victory in 1757 under Robert Clive against the forces of Bengal. It would mark the beginning of the British conquest of India

For further reading check the following articles:

• Battle of Plassey
• Robert Clive 
• European Colonialism