It's a slang, but originally it means 'tin' (plate), 'sheet metal'.
Nowadays it's a popular, quite vulgar slang among Russian youth; "жесть" (zhest') is something surprising, frightening, puzzling, shocking; any strong emotion of 'no comments' kind.
F.e.
- Сегодня в московском метро был очередной теракт.
- Жесть...
(- Today there was another act of terrorism at the Moscow metro.
- Zhest... ("It's tough"))