Two meanings:
1:A person 'drug addict' that would do anything for
a hit/high.
Desperate times call for
desperate measures.
Meaning everyone goes to the toilet, so perhaps someone had drugs In there system.
2:a person who uses
sexual acts as a way to get through life.
1:
"He'd suck the septic tank if he could"
"Have you seen that guys mum, so
feral she'd suck the septic tank if she could!"
2:
"How did she get that
promotion? Don't worry she'd suck the septic tank if it meant getting
ahead"