Definition
Someone with the roles of judge, jury and executioner; someone with full power to judge and punish others unilaterally.
See Also
How People Actually Use It
A commonly used idiom that refers to someone who takes on all three roles in a situation. This person not only makes decisions about guilt or innocence but also carries out the punishment themselves.
"Jake: Listen here, Emma. I’m your counselor, therapist, personal shopper, life coach, and if need be, your Judge, Jury, and Executioner Emma: Uhh... did you just say 'executioner' and 'therapist' in the same sentence? Jake: I like to cover all bases. Keeps things interesting"
Source: Urban Dictionary