Green’s Dictionary of Slang

Christmas tree n.3

[its triangular shape, plus the serrated edges filed into the knife are supposedly reminiscent of the stereotyped shape of the Christmas pine]

(US prison) a makeshift, home-made knife.

Show quotations
[US](con. 1998–2000) J. Lerner You Got Nothing Coming 65: ‘This here’s what we call a Christmas tree.’ The triangular shank’s base and center had been filed into a series of jagged, serrated edges, tapering gradually into an ice-pick point.