English is strange!!!!!

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i am confused by slangs.can anyone explain them for me? why"kick a s s"means great?why lemon means low quality?monkey equals 500$?why goose is stupid?suck means terrible?why white lie is a little lie and yellow dog mean betray,black sheep means a badsheep?what is the relationship between toilet and john?is there any source?i think Jhon is a name of man,just like D i c k. does the music is really unpleasure to face?"face the music" why "fat chance"is a little chance instead of big chance? here is a sentence,"what eating you today?"the translation is"What is bothering you?".why?