Meaning of an idiom-Please

Potshot -- one word, not two -- is an American idiom that dates from the 1850s. Its original meaning was a "shot taken at animal simply to kill for food" -- in other words, to get it in the pot, not for sporting or marksmanship. The original usage carried a sense of "opportunistic shooting". Gradually, the sense of "opportunistic" (and lack of marksmanship) began to prevail, and the sense of providing "food for the pot" declined.

The boys, proud of their new guns, took potshots at Old Man Miller's mailbox.

In the 1920s, the sense of "opportunistic criticism" developed, and has become its primary meaning today. Besides the being opportunistic, the criticism may be -- like the boys taking random shots at the neighbor's mailbox -- inaccurate. When you take potshots at somebody, you are hoping that one of your comments "will strike a nerve".

Even more common than Tolley's "talking junk" is "talking trash". However, this carries a sense of being boastful while also disparaging one's opponent.