A dead metaphor has transitioned from being a metaphor to being a sequence of tokens with a conventionalized meaning that might not have all that much to do with the original meaning.
Deadline and Balls to the Wall are classic examples. Many more know what they mean than know their origin.
For more, see Dead vs. Poetic Metaphor.
I'm skeptical about the practical usefulness of the idea. See Kinds of Metaphor.
DOT strict digraph rankdir=LR node [style=filled fillcolor=lightyellow penwidth=3 color=black fontname="Helvetica"] HERE NODE node [style=filled fillcolor=lightblue] WHERE /^Next/ LINKS HERE -> NODE node [style=filled fillcolor=white] HERE NODE WHERE /^Next/ LINKS HERE -> NODE node [style=filled fillcolor=white penwidth=3 color=black] LINKS HERE -> NODE node [style="filled,rounded,dotted" fillcolor=white] edge [style=dotted] HERE NODE BACKLINKS NODE -> HERE STATIC strict digraph {rankdir=LR node [style=filled fillcolor=lightyellow penwidth=3 color=black fontname="Helvetica"] "Dead Metaphor" node [style=filled fillcolor=lightblue] node [style=filled fillcolor=white penwidth=3 color=black] "Dead Metaphor" -> "Deadline" "Dead Metaphor" -> "Balls to the Wall" "Dead Metaphor" -> "Dead v. Poetic Metaphor" "Dead Metaphor" -> "Kinds of Metaphor"}