Something that happens willy-nilly is random or haphazard. If you put your clothes away willy-nilly, it'll take forever to find two matching socks.
This adverb is perfect for describing something done without any planning, like when the neighborhood kids grab instruments and parade willy-nilly up and down streets and through people's backyards. Willy-nilly comes from the obsolete phrase will I, nill I, or "I am willing, I am unwilling." The original definition, "whether one likes it or not," gradually evolved into today's meaning. Along the way, variations on willy-nilly included "nilly-willy," "willing, nilling," and "William nilliam."