plural of HB

Initialism of Hansestadt Bremen. The German city of Bremen. (Used on licence plates and informally in other contexts.)

hard black (describing a pencil that makes lighter marks than a grade B, but darker marks than a grade H pencil).

(stars) Initialism of horizontal branch. / (slang, seduction community) A hot babe, someone pickup artists try to seduce. (Usually followed by a number in the 1-10 scale of physical beauty, e.g. HB10.)