Throughout the Mahabharata, one of the most powerful and mysterious characters is Sri Bhishma, whom everyone addressed as Pitamaha i.e. Grandfather since he was expert and well trained in the science of milliatary warfare.

Bhishma pitamaha was earlier known as Devavrata. Bheeshma pitamah took an oath by the virtue of which, he got the title of Bheeshma and later on, pitamaha was appended when he showed his extraordinary skills of warfare.

He is the son of Ganga Devi and Maharaj Shantanu, who in previous life was cursed by Rishi Vashishtha to be born in the mortal world due to his attempt to steal his kamadhenu cow.

He got his name due to the terrible oath of lifelong celibacy which he took for the pleasure of his father. Once Maharaja Shantanu wished to marry a girl, Satyavati but her father had the condition that the sons of his daughter should be the rulers and not his other sons.

When he heard this, maharaj became very morose and then, seeing the long face of his father, he took the terrible oath of lifelong celibacy and serving the ruler on the throne. Being pleased upon him, Maharaj Shantanu blessed him with ichchha mrityu, the capacity to leave body whenever desired.

Bhishma grew up under the expert guidance of many sages like Brihaspati, Markandeya, Shukracharya etc. and learned many things from them.

In Srimad Bhagwatam (6.3.20-21)