Smelly means having a strong bad smell. Smelly socks, a smelly bin, it is smelly in here.
It is informal and always negative — nothing pleasant is described as smelly. The neutral word is scented and the positive one is fragrant. Note smell, which is neutral as a noun and negative as a verb — it smells means it smells bad, while it smells of lavender is fine. That asymmetry catches learners out constantly, and there is no rule for it beyond usage.
1adjectivesmellyHaving a strong bad smell.
“The bin has been smelly since about Tuesday.”
synonyms: stinking, malodorous
smelly is a quality of bad smell.
Smell appeared in Middle English with no known ancestor, which is remarkable for one of the five senses. Old English used stincan, which now means only to smell bad.
Proto-Indo-European
*(s)meld-
Middle English
smellen
English
smell
English, today
smelly
Built from
From smell + -y. smell comes from Middle English smellen. From Middle English smellen, smillen, smyllen, smullen, from Old English smyllan, smiellan (“to smell, emit fumes”), from Proto-West Germanic smallijan (“to glow, burn, smoulder”), from Proto-Indo-European smel- (“to burn, smoke, smoulder; tar, pitch”). The noun is from Middle English smel, smil, smul (“smell, odour”). Related to stq smeele (“to smoulder”), Middle Dutch smolen (“to burn, smoulder”) (whence Dutch smeulen (“to smoulder”)), Middle Low German smölen (“to be hazy, be dusty”) (whence Low German smölen (“smoulder”)), Low German smullen (“emit smoke”), vls smoel (“stuffy, muggy, hazy”), Danish smul (“dust, powder”), Lithuanian smilkyti (“to incense, fumigate”), Lithuanian smilkti (“to smudge, smolder, fume, reek”), Lithuanian smalkinti (“to fume”), mga smál, smól, smúal (“fire, gleed, embers, ashes”), Russian смола́ (“resin, tar”). Compare smoulder, smother.
Etymology from Wiktionary, CC BY-SA