Sentence:  Mr. Lamb starts thither with a big wallet and little <WSD>brainlet</WSD> to do a “good lot of thinking;” despoiled of one and nearly so of the other, he succeeds in doing a “lot of good thinking—for experience teaches him “what might have been done.” 
Meaning of the word marked with <WSD> tags:  A small brain; the brain of someone or something small or of someone who is not very intelligent.