Bait is a an internet slang term used to describe comments or opinions which are considered to be made purposefully to troll other posters or to start a flame war. The term is commonly found on message boards and in comment sections, including the 4chan imageboards.
In the 1990s, the term "flamebait" was used on Usenet to describe posts made specifically to incite an argument. One of the earliest examples of this was posted to the rec.autos.tech newsgroup on August 20th, 1995, in response to a complaint that American-made cars used soft suspensions and automatic transmissions that made Americans look weak and incompetent. After the poster went on to complain that the United States was spending money that they thought should have been allotted for healthcare and education on the military, another user referred to the comments as "flamebait."
"Flame bait" was added to the Free On-Line Dictionary of Computing on May 27th, 1998, defining the term as a Usenet post that is intended to elicit hostility. It was similarly defined on Urban Dictionary on March 24th, 2004. By September of that year, discussions self-identifying as "flame bait" had begun to appear on message boards as well. In 2005, the tech site Search SOA and the English education blog Antimoon (shown below) compared the differences between flame baiting and trolling, noting that the actions were often quite similar but fueled by different motives.
This Is Bait
As early as May 2013, an image macro featuring a royalty free clip art image of a fish chasing a hook with the caption "this is bait" began circulating 4chan as a method of drawing attention to baiting posts. One of the earliest archived uses of the image was posted to /a/ (Anime and Manga) in response to a post speaking negatively about the character Kirino Kousaka from the Japanese light novel series Ore no Imōto ga Konna ni Kawaii Wake ga Nai. The image and photoshopped derivatives of it have since appeared on a number of other 4chan boards including /vg/ (Video Game Generals), /tg/ (Traditional Games) and /s4s/ (Shit 4chan Says). "This Is Bait" images have also been found on DeviantArt and Ponychan.
 Democratic Underground – Tonight's flame bait: I CAN'T FUCKING STAND KRAFTWERK'S "AUTOBAHN!"
 Yahoo! Answers – What's the meaning of this: "gr8 b8 m8 check my 8"?