noun ( BrE , informal )
1.
[ C ] the last part of a cigarette that is left after it has been smoked
2.
[ sing. ] the ~ of sth the last part of sth, especially when it is less important or interesting :
I only caught the fag end of their conversation.
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WORD ORIGIN
early 17th cent. (in the sense remnant ): from 15th-cent. fag a flap , of unknown origin. The current sense dates from the early 20th cent.