not wanted or approved of; likely to cause trouble or problems undesirable consequences/effects It would be highly undesirable to increase class sizes further. prostitution and other undesirable practices These alternatives are economically undesirable. opposite desirableExtra examples It was felt that big cars were socially and economically undesirable. It was felt that the ageing of society was socially and economically undesirable. Why is he mixing with socially undesirable types? a situation which is highly undesirable She launched a personal campaign against all those she considered to be ‘socially undesirable people’. What is the best way to deal with undesirable behaviour?