Headmaster is a person who is extremely proficient in luring girls in (of any shape and size) to receive head. Also known as fellatio for you virgins out there.
However do not make the mistake to assume the headmaster is not a virgin. In most cases, the Headmaster is almost always a Virgin.
Monzy: Oi i got head from Raveena last night. She nearly ate my dick off
The Boys: Raa! Naa, how'd you pull her? You're defo the headmaster akhi!
The act of soliciting for the purposes of prostitution, i.e. a girl "working" in a bar, usually with unnecessarily revealing skirts and tops.
Has its origins in South London; etymology rumoured to be an oft-absent headteacher caught soliciting in the late 1970s. In order to avoid direct reference, the claim that he was doing his 'duty' evolved into 'headmaster's duty'.
TransFormers which have a Nebulan (humanoid) companion who transforms into the head of the transforming robot, and does not form part of (but can usually drive or otherwise connect to) the large robot's vehicle and/or beast mode/s.
Sometimes used to mean the large robot, sometimes the small figure/head. (Japan only: some heads were brought out without bodies, and all heads were interchangeable between robots, making more sense of the term "headmaster").
Of course, it also means a male head-teacher (now rarely used for PC reasons) - presumably it's a play on words (used as a model for later names such as Powermaster, Targetmaster etc.). Initially appeared in series 4 of generation 1.
Also, the first series of the Japanese Transformers cartoon (the 4th series overall) is called "TransFormers Headmasters".