Shitty weed that is watered down and you need like an 8th just to get high. It is very common in the Mexican culture mostly the illegal ones because it is cheap and easy to get. Another name for 30's is 40's which is even worse because its more watered down and sometimes 30's/40's is mixed with oregano because dealers who sell 30's/40's are trying to rip you off because they think your a cheap Mexican who just wants to get high. 30's is shwag weed.
Stoner 1: Yo! I bought some 30's from my dealer, that shit was hella cheap like 10 bucks a gram!
Stoner 2: We gonna get fuckin' ripped tonight! That looks like some dank ass nugs!
Stoner 3: Uhh..you guys are stupid 30's is shitty and it doesnt get you high.
Stoner 1: For real! Man this sucks! Fuckin' Mexican dealer ripped me off!
Stoner 2: Yeah that illegal immigrant trying to sell people shit weed, fuck him!