adverb
Describes a thick, sticky substance clinging to a surface, often with a negative or unpleasant nuance. Commonly used with verbs like つく, 塗る, or こびりつく.
パンにバターがべっとりついている。
Butter is thickly spread on the bread.
汗でシャツがべっとり肌に張り付いた。
My shirt stuck stickily to my skin with sweat.
adverb
cling closely; stick to
Emphasizes the state of being closely attached, often with a sense of unpleasant persistence. Used for things that adhere firmly and are hard to remove.
泥が靴にべっとりくっついて取れない。
Mud is stuck fast to my shoes and won't come off.
The word is likely a mimetic adverb (擬態語) derived from the sound or sensation of something thick and sticky. The exact origin is uncertain.