The story
From smack + -less. Compare Middle English smakkeles, smechles (“savourless; lacking appeal or gusto”), West Frisian smakeleas (“tasteless”), Dutch smakeloos, smaakloos (“tasteless”), German geschmacklos (“tasteless”), Danish smagløs (“tasteless”), Swedish smaklös (“tasteless”), Icelandic smekklaus (“tasteless; indecent”).
Etymology from Wiktionary, CC BY-SA 4.0.