1983, Edda: A Collection of Essays (Robert James Glendinning), page 177:riverVán in SnE I 21 is mentioned as coming from the slather of the bound Fenris Wolf.
1913, Joseph C. Lincoln, Mr. Pratt's Patients Chapter 1, Then there came a reg'lar terror of a sou'wester same as you don't get one summer in a thousand, and blowed the shanty flat and ripped about half of the weir poles out of the sand. We spent consider'ble money getting 'em reset, and then a swordfish got into the pound and tore the nets all to slathers, right in the middle of the squiteague season.
1919, Lucy Maud Montgomery, Rainbow Valley, ch. 24,In her eyes the manse people were quite fabulously rich, and no doubt those girls had slathers of shoes and stockings.
Verb
(transitive) To spread something thickly on something else; to coat well.I slathered jam on my toast.
(transitive, often followed by with) To applygenerously upon.I slathered my toast with jam.