Following a stupid comment the personwho said it must bend at the neck and look at the floor. thus putting their head down in an act of shameful embarrasement. After this the person cannot be made fun of.
Acronym: Head Down Mother Fucker! Typically used to indicate that either oneself or the recipient should put their head down and do some work.
Worker1: So worker2 how was your weekend?
Worker2: Good thanks. You?
Worker1: Oh fine. Hows the kids?
Worker2: Yeah pretty go-
Boss: Head DownMother Fucker!