Green’s Dictionary of Slang

H.M.I.C. n.

[head motherfucker in charge]

(US) the dominant figure in a given situation or institution.

[US](con. c.1970) G. Hasford Phantom Blooper 28: ‘I may be a slick-sleeved buck private to you, but I’m still H.M.I.C. around here.’ ‘H.M.I.C.?’ ‘Head Motherfucker in Charge.’.