Two important things things that you - and other learners - need to know: 1. The grammar of the word 'slang': It is an uncountable noun, so it is not possible to say 'a slang' or 'these two slangs'. 2. The meaning of the word 'slang': This is very informal language used by a particular group of people ( for example, the choice of language which soldiers or skateboarders use among themselves and that other people would not use). 'Lay a hand' is a standard expression which any native speaker might use, and it is NOT slang. You could call it a phrase, an expression, or - as it uses words in a non-literal sense - it could also be called an idiom. Finally, in answer to your question, I don't think that there is a significant difference between these two idioms. "I swear that I never laid a hand on her!" and "I swear that I never laid a finger on her!" both mean that the speaker didn't touch or harm the other person.