heavily (adv.)youdaoicibaDictYouDict[heavily 词源字典]
Old English hefiglice "violently, intensely; sorrowfully; sluggishly," from hefig (see heavy) + -ly (2).[heavily etymology, heavily origin, 英语词源]
slowly (adv.)youdaoicibaDictYouDict
Old English slawlice "slowly, sluggishly;" see slow (adj.) + -ly (2).
sluggish (adj.)youdaoicibaDictYouDict
mid-15c., from Middle English slugge "lazy person" (see sluggard) + -ish. Earlier adjective was sluggi (early 13c.). Related: Sluggishly; sluggishness.