Equal parts offensive and absurd, he is the standard bearer for British comedy, loved by comedians, hated by the right wing and ignored by the public.
I think one reason he never got big in America is the specific UK pop culture references,so Im going to try and leave that stuff out or explain it as much as possible.
Top Gear, so you'll know what he's going on about to start with:
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A ridiculous thing about 'office world man'
This is one of Stewart Lee's favourite types of comedy,taking something that isn't funny and continuing on with it for so long that it becomes funny.
(This is from a show called 'Carpet Remnant World' which explains one of the jokes in it)
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Anti-Islamic standup
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Mocking right wing views on immigration (UKIP are a right wing anti-immigration UK political party led at the time by Nigel Farage and Paul Nutall)
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BREXIT HAHA (lots of specific UK politics references here)
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If you like this stuff, some of his full standup shows are on Youtube, I would highly recommend watching them for the full experience (Content Provider and Carpet Remnant World are really good)