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 claimed that slacker comes from the Irish sleabhcadh, which means to go limp or to wilt, or the Scottish Gaelic slabhcar, meaning a slouching person. The words sleabhcadh and slabhcar don’t sound much like slack. They are pronounced to rhyme with cow (or sometimes as low, depending on dialect), as shlowkoo or slowkar. They may be cognates of the English slack but they are not the origin of it.
There is absolutely no doubt about the Germanic origins of the word slack or its antiquity in English. The word slack is part of the basic Germanic vocabulary of English. It goes back to Old English (the ancient version of the language used before the Norman Conquest) and is very well attested. The OED says that it is from “Old English slæc ‘inclined to be lazy, unhurried’, of Germanic origin.” Slæc was pronounced the same as modern English slack. In Middle English, it was written slac. For example, here is our old friend, the Michigan University Middle English Dictionary:
Slac (a) Of persons: indolent, lazy, lax; negligent, remiss; slow (to do sth.) …
Slacker seems to be a development from these meanings of slack in the late 18th century.