For some time now, some of my on-line friends have advised me to provide a version of CassidySlangScam without the invective aimed at Cassidy and his supporters. In response to that advice, I am working on providing a glossary of the terms in Cassidy’s ludicrous book How The Irish Invented Slang with a short, simple and business-like explanation of why Cassidy’s version is wrong.
There is nothing mysterious about the origin of the word sketch. By the 1660s, it was found in English as a word for a rough drawing. It comes from Dutch schets or Low German skizze, both apparently borrowings from Italian schizzo “sketch, drawing.” By 1789, it had acquired the meaning of a ‘short play or performance, usually comic’.
By the 19th century, a sketch had come to mean something or someone very funny.
Daniel Cassidy, in his etymological hoax, How The Irish Invented Slang, decided that this had to come from Irish, so he made the claim that it derives from the word scairt, meaning shout. There is no evidence for this, and the word scairt is pronounced skartch. Obviously, a derivation from sketch as in picture makes more sense than it deriving from the Irish scairt.