Douglas Harper's Etymology Dictionary
also pitter-pat, 1520s; imitative. As a noun from 1580s.
Wiktionary
adv. With a quick tapping sound n. A series of quick tapping sounds vb. To make a series of quick tapping sounds; to palpitate
WordNet
adv. as of footsteps; "he came running pit-a-pat down the hall" [syn: pitty-patty, pitty-pat, pitter-patter]
describing a rhythmic beating; "his heart went pit-a-pat" [syn: pitty-patty, pitty-pat, pitter-patter]
Usage examples of "pit-a-pat".
When he thought of all that Blinky said, his heart went pit-a-pat so loudly that he became frightened at the very thought of running away.
Then we heard their quick pit-a-pat along the quiet little street as we put on our calashes and pinned up our gowns.
Wilton,” said that charming dame, “my heart goes pit-a-pat when I see you, for it's almost like being among those dreadful highbinders again, and how could you bring the horrid creatures down on our dear Luella, when she might have been captured and sold into slavery under our very eyes.