Do you know ghouls from goblins and ghosts?
mid-14c. alteration of slumeren (mid-13c.), frequentative form of slumen "to doze," probably from Old English sluma "light sleep" (cf. Middle Dutch slumen, Dutch sluimeren, German schlummern "to slumber"). Frequentative on the notion of "intermittent light sleep." For the -b-, cf. number, lumber, chamber, etc. Related: Slumbered; slumbering.
mid-14c., from slumber (v.). Slumber party first recorded 1942. Slumberland is from 1875.