Molly whop is a slang for getting hit hard, usually with fists, but can be extended to include feet, objects, etc.
To molly whop someone means to beat someone up or to slap someone hard.
Idiom is an expression in the usage of a language that is peculiar to itself either grammatically (as no, it wasn't me) or in having a meaning that cannot be derived from the conjoined meanings of its elements (as ride herd on for “supervise”)
Every language has its own collection of wise sayings. They offer advice about how to live and also transfer some underlying ideas, principles and values of a given culture / society. These sayings are called "idioms" - or proverbs if they are longer.
These combinations of words have (rarely complete sentences) a "figurative meaning" meaning, they basically work with "pictures".
Here’s a list of commonly used idioms and sayings (in everyday conversational English), can help to speak English by learning English idiomatic expressions. This is a list, which contains exactly 66 of the most commonly used idioms and their meaning.