Light weight

[Sonia]

Light weight is getting drunk really fast

[Jessica]

One who fails at drinking.

[PussyCat]

Person who gets drunk or high really easily and doesnt need much of it.
Examples:
Jay: Jen got drunk off of three tequila shots.
Jo: What a light weight.

Fred: Dude, you only had, like, two hits. Youre such a light weight.