the.com/mixed metaphor

means A figure of speech that jams together two or more incompatible images or comparisons in one statement, producing nonsense instead of vivid sense.

from The pieces are plain English: 'metaphor' traces back through Latin to Greek metaphora, 'a transfer' or 'carrying over,' from metapherein (meta- 'over' + pherein 'to carry') — the basic act of carrying meaning from one thing onto another. 'Mixed' is just the ordinary word for blended or combined. The phrase itself seems to have arisen naturally in English as writers and critics needed a label for the common slip of yoking two unrelated metaphors together — no single coiner or dramatic backstory is recorded; it's a descriptive compound that named a familiar blunder once people started noticing it in print.