There is no formal definition of unreasonable behaviour.
Violence to the petitioner or to the children of the family is one example.
Excessive drinking, excessive work commitments, constant nagging, financial irresponsibility (for example, the build up of debts to the detriment of the family), and refusal to have sexual intercourse are further examples which can be cited in a petition of unreasonable behaviour.
You should discuss your spouse's behaviour with your solicitor who will advise you how this should be dealt with in the petition.
It is usual to make one or two general statements about the respondent's behaviour and to give specific instances of the respondent's unreasonable behaviour, giving brief details, dates and places.
You must prove, to the satisfaction of the court, that the respondent has behaved in such a way that you, as the petitioner, cannot reasonably be expected to live with him or her.
If you and your spouse are still living under the same roof it is important to cite the most recent instances of unreasonable behaviour, certainly within the last six months.