Have you ever noticed that there aren’t any words in French or Spanish that begin with sl-? There weren’t any in Latin, either. Every language rules out certain combinations of consonants, as being too hard to pronounce. Hawaiian rules them all out! You never get two consonants together in Hawaiian, but you sure get a lot of vowels to make up for them.
Now then, we know that the English language is a cousin of French and Spanish—and Latin. Either Latin lost all the wordsthat survive in English beginning with sl- (and in German, beginning with schl-), or the words are there, but they’re hidden. It’s the latter. Latin speakers didn’t like the sl, just as in Middle English we stopped liking kn- and wr-, ending up pronouncing only n and r. The Latins dropped the s.
The root idea underlying the sl- words is that of fluidity or softness or weakness: slow, slug, slink, slick. The Latin relatives of our sl- words begin with l-. So slack is related to Latin languere, to languish, to lie about, to be lax (from the past participle,laxus). So a slacker is lax, by definition! We see a similar doublet in this sentence: I saw the liquid cat slink into the basket.