Being thus addressed by Hanuman, Bhima proud of the strength of his arms, took him for one wanting in energy and prowess, and thought within himself, ‘Taking fast hold of the tail, will I send this monkey destitute of energy and prowess, to the region of Yama.’ Thereat, with a smile he slightingly took hold of the tail with his left hand; but could not move that tail of the mighty monkey. Then with both arms he pulled it, resembling the pole reared in honour of Indra.

In plain words: Bhima thinks the monkey is weak. He smirks and grabs the tail with one hand, planning to fling him away, but he cannot move it. Then he pulls with both arms.