Slumber

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Middle English: alteration of Scots and northern English sloom, in the same sense. The -b- was added for ease of pronunciation.

wiktionary

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From Middle English slombren, slomren, frequentative of Middle English slummen, slumen(“to doze”), probably from Middle English slume(“slumber”), from Old English slūma, from Proto-Germanic *slūm-(“slack, loose, limp, flabby”), from Proto-Indo-European *(s)lew-(“loose, limp, flabby”), equivalent to sloom +‎ -er. Cognate with West Frisian slommerje, slûmerje(“to slumber”), Dutch sluimeren(“to slumber”), German schlummern(“to slumber, doze”).

etymonline

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slumber (v.)

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.

slumber (n.)

mid-14c., from slumber (v.). Slumber party is attested by 1942. Slumberland is from 1875.