also: ウマなみ
na-adjective, noun
hung like a horse; big-dicked
Vulgar slang describing a man with a very large penis. Often used in casual, crude, or joking contexts.
あいつは馬並みらしいよ。
I hear that guy is hung like a horse.
馬並みって言われたことある？
Have you ever been told you're big-dicked?
巨根 is a more neutral, descriptive term for a large penis, while 馬並み is slangy and explicitly compares to a horse.
Compound of 馬 (uma, 'horse') and 並み (nami, 'average, level'), literally 'horse-level'. The slang meaning derives from the notion of being as well-endowed as a horse.