Something that is disadvantageous to you puts you in a worse position than other people. The Second World War started in the most disadvantageous possible way for the western powers. = unfavourable advantageous. unfavourable and likely to cause problems for you advantageous
Attended with disadvantage; unfavorable to success or prosperity; inconvenient; prejudicial; opposed to advantageous; as, the situation of an army is disadvantageous for attack or defense