Murtaugh's catch phrase in these movies was "I'm too old for this shit"
Putting something on the Murtaugh List acknowledges that as you grow up, there's stuff you just can no longer do that you did in your youth.
BARNIE: "Hey Ted, want to dye your hair a funny color and attend a rave?"
TED: "No Barney, I can't. It's on my Murtaugh List"