"Shoot bull" is a colloquial expression that means to engage in idle conversation or talk aimlessly. There are several other synonyms that can be used instead of this phrase. One of them is "shoot the breeze" which is also used to describe a casual conversation. Another is "chew the fat" which means to talk casually or leisurely. "Gossip" is another synonym to describe idle chit-chat or talking about others. And finally, "jabber" is a more informal and often negative term that describes meaningless or endless talking. Regardless of which synonym you choose, they all convey the act of engaging in non-productive conversations with others.