Diminutive of "my bad", the enunciation of which saves neither the enunciator nor the
listener any time of mind or eardrum, though which does indeed spare the enunciator the torment of touching the tip of his tongue to the front upper roof of the mouth as when one does
actually say "bad".
Yo 'sup?! I lament the laze and
malaise of the
muthafucka who can't bring himself to say "my bad" but
instead must employ "my b" in its stead, even though there is no contractive benefit gained by the effort, muthafuckas!