What is a GSLV fat boy?

GSLV Mk III, the heaviest rocket built by ISRO was nicknamed the GSLV fat boy. You can read about the GSLV Mk III – A Comprehensive Overview of Heavy Lift Launch Vehicle in the given link.

Further readings:

1. Mars Orbiter Mission (MOM) – Payloads, Launch Vehicle, Achievements
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)