backdoor
1. adjective Done or achieved in a clandestine, stealthy, illegal and/or illicit way, or by such means. Such backdoor appointments of its chief members of staff make me worry that nepotism is rife within the company. They were arrested by customs agents for their backdoor import of cheap tobacco into the country.
2. verb To use deceitful, duplicitous, or morally questionable means to circumvent someone's authority, influence, or integrity for the sake of one's personal gains. Government agents tried to backdoor the activist by planting discrediting information about her in newspapers.
backdoor man
A man having an affair with a married woman. I can't believe you're having an affair with a married woman! That makes you a dirty, rotten backdoor man!