Dap: A fist-to-fist greeting, conducted as though the two parties are punching each other lightly. The word is both a noun and a verb; giving dap is also acceptable. The gesture made headlines and occasioned earnest analysis when Barack Obama and his wife, Michelle, bumped knuckles after the candidate's June 3 victory speech in St. Paul.
Jill Rosen, writing in the Baltimore Sun, traces the dap back to the black-power movement in the 1960s, and says the word is an acronym for dignity and pride. However, another meaning of dap goes back at least to the 1950s, when it was an abbreviation for dapper. The Random House Historical Dictionary of American Slang (RHHDAS) gives this citation from 1956:
The word now is dap. You want somebody to know a man is sharp, is au reet, you say he's dap.
("Au reet": hepcat slang for "all right.")
That meaning persisted at least through the 1970s, when the "greeting" meaning began to be adopted. This Vietnam War glossary defines dap as "an elaborate, stylized handshake used mostly among black Americans in Vietnam." RHHDAS provides a 1983 citation asserting that dap means "beautiful" in Vietnamese.
A third meaning of dap is "credit where credit is due," also known as "props." Juba to Jive: A Dictionary of African-American Slang dates this usage to 1990s "southern and northern city use."
Back to the Obamas. Time magazine called the greeting a "fist bump," but noted that:
[T]he terminology used to describe the manual move is under dispute. On reporting Obama's speech, The New York Times described it stuffily as a "closed-fisted high-five" while Human Events racily suggested it was closer to "Hezbollah-style fist-jabbing,"(the phrase was later removed from the article). One Internet poster even referred to it as "the fist bump of hope." Other terms for the move include "power five," "fist pound," "knuckle bump," "Quarter Pounder" and "dap."
Black folks give each other dap in all sorts of instances--not just competition. I may give my man a "fist-bump" (arrrgggghh!!! it burns!!!) because my fingers are greasy, because I've got something in my hand, or just because I feel like it. In fact, one of the more awkward moments in black life occurs when two brothers greet each other and one isn't sure whether to use the open hand or the closed fist. You can end up with some pretty awkward exchanges--like shaking a dude's fist.
(Photo from Time. Hat tip for the Coates link: Davos Newbies.)