uhhh what?
吹牛 does not break down to (blow cow) syntax-wise.
while the 吹 does mean (blow) in terms of (blowing it out of proportion), if you want the 牛 to standalone in this case, it is actually shortened for 牛屄 which means (hardcore/fucking awesome).
吹牛, instead of (blow cow), would better translate as (blowhard).
well, MC prob didn't really blow all that hard actually~