i think any variation on "mercy" is kind of silly for a safeword (although i do understand what you mean about the "mister" part. that was dumb too). it reminds me of that childhood game where... goodness, what did the two kids do to each other? something painful, and you lost when you couldn't take it anymore and said "mercy." maybe this guy you were involved with always lost that game when he was a kid and needed some kind of activity to rebuild his self-esteem.
