Imperative verb phrase. According to the OED, “said (often parenthetically) to create an interval of suspense before imparting something remarkable or amusing, in order to heighten its effect. Also ironically.” Often doubled, as in an example cited by the OED from R. Laidlaw’s 1979 book Lion is Rampant: “The real attack will come from, wait for it, wait for it—anither direction a’thegither.”
It has been popularized recently by Barney, the character played by Neil Patrick Harris (left) on the American sitcom “How I Met Your Mother,” one of whose verbal ticks is to infix wait for it in the middle of words. For example: “Our friend Robin used to do porn—wait for it—ography!”
When Karl Baedeker (or, more likely, one of his minions) passed this way [Phoenix, Arizona] in 1904, he found ”a well-built, modern city” of . . . wait for it . . . exactly 5,554 inhabitants. (R.W. Apple, New York Times, February 19, 1999)/Judging by my inbox, a large proportion of angry white men also believe that the burst housing bubble and financial meltdown of 2007-8 were caused by — wait for it — President Jimmy Carter. (Salon.com, April 29, 2011)
notoneoffbritishisms.com 
The origin of the phrase lies in military usage, or supposed military usage: an NCO, issuing instructions to troops on parade, will pause before uttering the final word of command – the point is to react to the word of command, rather than out of habit. “By the left – wait for it – *march*!” I don’t know whether this was ever really the practice in the British army, but it is a common caricature in popular culture. I think “Wait for it” was elevated to a catchphrase in the 1970s by Windsor Davies, who played a sadistic sergeant-major in the (dreadful) Second World War-set sitcom It Ain’t ‘Alf ‘Ot, Mum.