Green’s Dictionary of Slang

weak adj.

1. [1950s] lacking in funds, poor.

2. [1950s+] poor, disappointing, ineffectual.

3. [1970s+] stupid.

4. [1990s+] (US campus) frustrated.

In compounds

weak-ass (adj.) [-ass sfx]

[1950s+] a derog. term for second-rate, unimpressive, powerless.

weak shit (n.) [1960s+]

1. (drugs) second-rate, relatively ineffective drugs [shit n. (5c)].

2. second-rate, weak, inadequate words or actions [shit n. (3e)].

SE in slang uses

In derivatives

weakie (n.) (also weaky) [sense 2 + sfx -ie]

[1940s+] (Aus.) an unreliable, untrustworthy person.

In compounds

weakheart (n.)

[1970s] (UK/W.I.) a derog. term for a police officer.

weak-jointed (adj.)

[mid-19C] (US) euph. for drunk.

weaksauce (n.)

[2000s+] (US teen/campus) something or someone second-rate, tedious, specifically an unfunny joke.

weak tap (n.) [tap v.2 (3b)]

[1950s] one who has a low income.

In phrases

weak as gin’s piss (adj.) (also weak as a pygmy maiden’s piss, breath, ...nun’s piss, ...puppy’s piss, ....weasel’s piss) [gin n.1 (1) + piss n. (1)]

[1940s+] (orig. Aus.) extremely weak.

weak in the arm (n.)

see under arm n.