(Spanish Slang) loosly translated into English as "Cocksucker". The word literally translates into "Person who sucks"; therefore, it is only implied that you suck penis. A person described as a mamilon will usually be annoying or a kiss-ass.
Jose notices Juan kissing up to his supervisor. Juan was being a mamilon.
Maria was being trolled by Nacho. Not wanting to be bothered, she flatly told him "No seas mamilon."