...Valid words in the Bee include pitapat, ratatat, and gimme, among others that seem like slang, at best. But after doing hundreds of them you get to know the weird entries.

But it's in my Merriam-Webster's Collegiate with an origin date of 1656.

If you had asked me before this, I would have grouped it with other semi-accepted onomatopoetic words and would have spelled it with an apostrophe. Shows what I know.