Derived directly from a song by the same name performed by the Temptations, and frequently played on oldies radio stations in the United States. Usage is generally considered bad form, as typically it's anything except an emotion that the person is agreeing with.
It's basically a misuse of verbage from the Town Hall form of voting, also used in many clubs and other organizations, where someone makes a motion (suggestion), and someone else will second (agree with) it in order to bring the issue to a vote.
person 1: I make a motion to accept this issue with the proposed amendment.
person 2: I second that motion
chairperson: All in favor say aye
It's a mispronounciation that just stuck, since you might actually agree with the emotion behind the particular decision which is being discussed.
dude2: dude, i second that emotion