mid-14c. alteration of slumeren (mid-13c.), frequentative form of slumen "to doze," probably from Old English sluma "light sleep" (compare Middle Dutch slumen, Dutch sluimeren, German schlummern "to slumber"). Frequentative on the notion of "intermittent light sleep." For the -b-, compare number, lumber, chamber, etc. Related: Slumbered; slumbering.
mid-14c., from slumber (v.). Slumber party is attested by 1942. Slumberland is from 1875.