I second that 

The slang meaning is "I agree" or "I support that".

The real meaning comes from parliamentary procedure, the way in which a group of people come together and most efficiently present and discuss possible courses of action, and make decisions. "Roberts Rules of Order" has been the standard of our government, judicial system, and formal organizations since 1876.

One member may stand and address the chairman. Once recognized, the member makes the motion: "I move that/to..." and resumes his seat. Some types of motions require another member, without rising, to second the motion: "I second the motion," or "I second it" or even just "second." Once seconded, the motion becomes a topic of organized conversation until two-thirds agree to vote. Seconds are important because some topics are not worth the group's time to discuss and a 2nd person means the topic is important to more than one person. Technically, the 2nd does not have to support the motion, they are simply agreeing that it should be a topic of discussion.

Tune in to any congressional session on CSPAN or attend your city council meetings to see Roberts Rules of Order in action.

The Temptations song "I second that emotion" is a pun on Roberts Rules of Order.

"People should stop posting images over 200kb on the front page."
"I second that."

In this slang use, it means support of the statement but it stays just a complaint. However, if the board were using real parliamentary procedures, the seconded motion would invite others to post additional comments and would remain a live topic until a vote is called.

"I move that images on the front page be limited to 200kb."
"I second that motion."
"Dial-up users are burdened with file sizes that large when twenty or thirty of them must be loaded at once."
"People can put links to bigger images instead of the whole thing messing up the way the text flows."
"Only 5% of our members are dial-up; we should be able to post whatever we want for the majority of the users who are high-speed."
(Vote: aye/yay/yes/hands or no/nay/hands)
"Yays have it, motion is carried that images on the main page be limited to 200kb or less." (This is now a rule.)

I second that by Coell September 5, 2005

I second that.

a phrasal sentence that means, when taken literally, “I echo that statement of yours.”, “I echo your above statement.”; “I agree with you 100%ly.”; or simply “Ditto.”. See also "I second that emotion".

Phrase used frequently on message boards (more so than in person) by someone usually immediately after someone-else stated something with which the other person agreed.
Note that the word ‘second’ is being used as a verb in this sense, and is derived from ‘2nd’(as opposed to the length of time).

Person1:I personally like Kingdom Hearts better, because I hate turn-based battle system....Real time is better to me.

Person2: I second that^.

I second that. by Victor Van Styn August 28, 2005

I second that emotion 

"I agree."

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

<voting commences>

It's a mispronounciation that just stuck, since you might actually agree with the emotion behind the particular decision which is being discussed.

dude1: dude, we should totally hit taco bell
dude2: dude, i second that emotion

I second that emotion by Brew78 August 28, 2006

I second that emotion 

"I agree with you on that subject."

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.

When I told Clyde that the best thing for the cash was to be placed overnight in the safe, he said "I second that emotion". I promptly beat him over the head with a stack of one dollar bills.

I second that emotion by Dennis The Tiger May 3, 2005

I'm gonna second that 

When you finish making a sandwich and you go to put the condiments away, someone comes up behind you and says "wait I'm gonna second that!"

Basically when someone saves you from having to out something away.

Sweet relief.

"Hey man don't put the peanut butter and jelly back I'm gonna second that"

I'm gonna second that by parisfranke September 30, 2008