A phrase that came into minor fashion among Daria fanfiction writers in late 2007. First used by Brother Grimace, 'Wait for it...' became a saying used by writers when readers of their fics became curious about plot developments in their works, or when readers wondered when new chapters of the fics would be posted.
The term is a 'Shout-out' to the character of Barney Stinson on the CBS television show How I Met Your Mother, who originated the term. Brother Grimace, a big fan of the show and of Barney Stinson, has confirmed that the phrase is an homage to 'an incredibly awesome, indeed, legen- wait for it - dary character.'
However, the term has often been used earlier than this, particularly in British comedy involving the British Army. Generally, this entails a commanding officer or sergeant beginning to give an order, then interrupting himself with the phrase in order to keep overenthusiastic soldiers under his command to keep from jumping the gun, as in:
"Ready...aim...wait for it...FIRE!"