I agree that it seems very likely 'slather' comes from lather Whitman, especially considering the Old English meaning of lather, 'to cover with'. Maybe 'lather' has been combined with 's' because so many words with similar meanings start with 's', e.g. slap on, slop. A portmanteau? An odd onomatopoeic invention from lather? Either way it's is great sounding word.


aside: Any Aussies remember 'Slip,Slap,Slop'