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"))

It relates to the adverb "жестоко" (cruel, harsh, brutal). Before "жесть" became common, we often used (and still sometimes employ) this fuller and more transparent form in similar contexts. Therefore, it would be wrong to interpret the modern colloquial exclamation "жесть!" through the noun "жесть" ("tin"). But I think it was exactly because of this contrasting unrelatedness that the new meaning became so popular.

People can use "жесть" when they are shocked or amused.

colloquially it means something cool or something terrible.what has caused a lot of emotions)))

Большое спасибо!