burner n. a gun, especially a handgun. (source: Double-Tongued Dictionary)
Further reading
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On HBO’s “The Wire” the term burner is used to describe prepaid cell phones the drug dealers use and discard when the minutes run out, to circumvent the possibility of wire-tapping.
That’s a good use of “burner,” a clear off-shoot of the old phreaking days when all cell phones were analog and it was easier to clone them. A “burner” then was a cell phone you “burned” with the new identifying information stolen from elsewhere, which you had to do because there were no SIM chips. There was a bit of reinforcement of the word because you “burned right through” those phones: you used them as long as you thought they were safe, then threw out the info and burned new info into the chip.
I see the connection between the handgun and cell phone. In both cases you throw them away after use so that they cannot be later found on you and traced back to the crime.