The expression that somebody has done something “for kicks” means that he or she has done it just for fun or for the enjoyment of the activity. Expressions such as this are idioms, which means that something is said in a way that the real meaning cannot be predicted just from the common meaning of the words used. Idioms from one region might be difficult for a person from somewhere else to understand. Often, idioms can prove extremely difficult to understand when they have been translated from another language. Phrases which are idiomatic often originate with a certain group of people or in a certain region; the phrase originated as an expression in the United States.
This phrase most likely originated from the jerky movement that one might exhibit when reacting to a strong electrical discharge or the firing of a weapon. The meanings of words and phrases often change over time in any language. A slang phrase or word that meant one thing 100 years ago might have an entirely different meaning now. The phrase, after originally being used to describe a quick and involuntary action, became associated with doing things just for the fun of doing them.
The concept can have a range of meanings. This idiomatic expression can have more of a positive connotation, as in someone who participates in sporting activities or helps other people “for kicks.” For example, a person might say that he or she mowed a neighbor's lawn just "for kicks."
The phrase also have can more of a negative implication if it is used to describe someone who commits crimes or does things that are wrong just for pure enjoyment — or “for kicks.” A teenage boy who is asked why he vandalized someone's property or stole a child's bike might explain that he did it just "for kicks." This idiom also can have a neutral connotation, as in someone who decides to skip stones at a lake or take a walk through the park just "for kicks." Such activities might be neither positive or negative, but someone might do them just to enjoy them.