"Unreasonable behavior" refers to actions or conduct that are not fair, logical, or acceptable in a given situation. It usually involves behavior that is difficult to understand or justifiable by others. Full definition
Instead, he will have to rely on grounds of unreasonable behaviour. (familyblawg.com)
Following a 39 year marriage, an affair by the wife, and being separated for two years, the wife's petition for unreasonable behaviour was refused. (dmhstallard.com)
It is a common misconception that matters such as unreasonable behaviour and adultery constitute «grounds» for divorce under English law. (vardags.com)