The original meaning of 「雑草{ざっそう}」 is, of course, "weed".
When used to describe a person, however, it refers to a non-star or non-elite type whose name no one knew at the beginning. The term is most often, if not exclusively, used to refer to athletes of mediocre ability.
Those types, however, occasionally end up very successful for their "weed-like" ineradicability.
Please note that it is not offensive to call oneself 雑草, but to call another person 雑草 can be.
"Even though he has been called 'the Sniper' with no matches whatsoever in Asia, that guy was originally a pretty run-of-the-mill boxer."
上原浩治{うえはらこうじ}, a notable baseball pitcher who has played both in Japan and the U.S. successfully, for instance, is known to write 「雑草[魂]{だましい}」 (literally, "weed spirit") alongside his name when he gives autographs.
「雑草魂」 even won the 流行語大賞{りゅうこうごたいしょう} in 1999.
Check out the video title here:
https://www.youtube.com/watch?v=GPGYpUseArU