How the word Sanyasa (सन्न्यास) was formed ?

सं + नि + अस् + घञ् ( प्रत्यय ) = सन्न्यास

सं + न्यास ( putting away ) = सन्न्यास 

This meaning is taken from Bhagavata Gita chapter 18 verse 2

काम्यानां कर्मणां न्यासम् सन्न्यास इति !

Giving up of actions motivated by desires is called सन्न्यास