“胆小”和“胆小鬼”这两个词是怎么来的？

Answer 1

胆小是形容人的性格，唯唯诺诺、谨小慎微。
胆小鬼是指一个人，唯唯诺诺、谨小慎微。
前者是形容词，后者是名词。

Answer 2

胆小的，胆小鬼 craven
一般口语里都用yellow表示胆小

He is yellow.
(他是个胆小鬼。)
He has a yellow streak in him.
(他胆小如鼠。)
When he refused to fight, his classmates taunted him:
“You are yellow, afraid to fight. You''ve got a yellow streak down your back a mile wide.“
(当他拒绝打架时，他的班上同学骂他「你是个胆小鬼，连打架都不敢打，真没出息。」)
【上文yellow streak是「胆小或胆怯的成份或倾向之意。」 】

“yellow“用来说人胆怯的词汇还有“yellow-bellied“(俚语)及“yellow-dog“:

George is a yellow-bellied guy, don''t send him on such a tough assignment.
（乔治是个胆怯的家伙，不要给他一份如此艰难的工作。)
“Why did you two boys fight “ the teacher asked in a severe tone.
「你们两个孩子为什么打了架呢？」老师以严厉的口气责问。
“ Well,“ said one, “he called me yellow-dog!“
「他骂我胆小鬼。」(其中)一个男孩回答。
“ Is that true “ questioned the teacher.
「真的吗？」老师问了(另一个男孩)。
“Yes, of course. He is a coward. Too yellow to fight like a man! “
「是呀，他真是个懦夫。他太没出息，不敢像个男子汉去打斗。」

Answer 3

因为胆会因为惊吓而破裂，如果你承受不了惊吓就叫做胆小了。

胆小鬼是民间幻想出来的胆小者的代言人。