Rule with an iron thumb

I heard this gem from” the master” as he described our boss: “You know, she rules with an iron thumb.”  This is a mash-up of “rule with an iron hand” (harsh leadership) and “under my thumb” (controlling someone).  Both phrases have similar meanings and both contain a similar body part so the mix-up is obvious.  Of course, “the master” also enjoyed The Rolling Stones so it is possible that the song “Under My Thumb” played a little part in his twisted but ingenious mind.  “My ol pal ” noted that “rule of thumb” (a general principle based on experiment) was also probably on ‘the master’s” mind and I agree.  This idiom has the words rule and thumb, so those words were swirling in his head for sure.