Is used to describe someone who is well-known for excessive talk which is usually boastful, exaggerated and unreliable.
You might think that "bullshit" is an ancient Anglo-Saxon word, but in fact it is a rather modern term.
The earliest example found by the Oxford English Dictionary is from around 1915, in a letter by Wyndham Lewis. Poets clearly like it. One of the earliest recorded examples of "bullshit" as a verb ("to bluff ... by talking nonsense") is from Ezra Pound's Pisan Cantos of 1949.
In British English it first became established as army slang, it is now mainstream English in the US and UK.