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what does "จังเลย" mean?

"จังเลย" it use for bulid the strong feeling of that word, stress the sentence that it is real and make the sentence feel prettier.

such as:

"คิดถึงเธอ" >>Missing you "คิดถึงเธอจังเลย" >> Really Missing you

however, you should not use by the formal sentence. It just the word to use an informal situation.

this word work as "quite"
quite good, ดีจังเลย
quite badแย่จังเลย
quite fast เร็วจังเลย
quite slow ช้าจังเลย
quite low ต่ำจังเลย

and you can say it short just "จัง" mutually work as "จังเลย"

hope it help!
Cheers!