你说在词典上没有查到，可能是你的词典不够“大”。你找一本稍“大”一点的词典，就可以查到的。the firstest with the mostest 是习语，为非正式用语，其英文解释是：the earliest and in the largest numbers, the earliest with more of what’s needed, first with the most，意思是“最早且带着（所需的）最多的东西”。如：

(1) Pete got the prize for being the firstest with the mostest. 彼特获了奖，是因为他最早带着最多东西到达那里。

(2) I always like to be there early—the firstest with the mostest. 我总是喜欢早早赶到那儿——到得最早，且带的东西最多。