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.
Cassidy claims that the African-American expression to dig (to understand or to like) comes from the Irish question form of the verb tuigeann, to understand. In the question form, this becomes An dtuigeann tú? (An deggan too?) This is not entirely impossible, though it is unlikely. Most etymologists think it comes from the English verb dig, as in ‘to make a hole’. This became student slang for studying, gaining an understanding of something by the 1830s. Then it became the black slang term for understanding in the 1930s. The meaning ‘to like’ came later.
Wherever it comes from, the word dig and its relationship to twig and tuig was being discussed by linguists a long time before Cassidy came along. Twig and its relationship to tuig was first discussed by Walter Skeat, who died in 1912. Both twig and dig and their possible origin from tuig were discussed in a paper by Eric P. Hamp, first published in 1981, called “On the Celtic origin of English slang dig/twig ‘understand’”. In other words, this claim has nothing whatever to do with Cassidy and predates his book by decades.