Where does “mann” come from?

mann (Middle Welsh) comes from Latin menda, from Proto-Indo-European mend- — physical defect, fault; to fault.

Ancestry of “mann”, step by step

Step	Language	Word	Meaning
1	Latin	menda	defect, blemish; mistake, error, blunder
2	Proto-Indo-European	mend-	physical defect, fault; to fault