—used in place of most (as in imitation of childlike speech)
… Americans are such suckers for TV commercials that we are powerless to resist the product that hits us with the mostest. Russell BakerThe disposition of personal property often becomes a scavenger hunt. Items are tagged while last breaths are being gasped. To the crudest goes the mostest. Thomas EastonLady Pamela Berry, wife of a newspaper proprietor and in her day the London hostess with the mostest, had given a dinner party in May 1951 that deserves both literary and historical footnotes. Geoffrey Wheatcroft