a reason for doing something, be it stupid or insignificant.
An act of impulse

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

doing things for fun/for a laugh
having fun
fuzies is what funzies does
etc. u get it

A mix b/w crazy and fun. Describes a funny crazy fool who will do anything to make someone laugh. Originated from a drunk Irish Man(Me).

A word from an Internet meme that originated in Pakistan, an amalgamation of the words fun and lolzy meaning to have fun and a laugh at the same time.

When you and your significant other are sexting and masturbating

Means doing something just for the fun of it, especially something illegal, perverse, pointless, stupid or dangerous.