IPA: //tɛˈsteɪtrɪks//
KK: /tɛsˈteɪtrɪks/
A woman who has created a legal document stating how her property and belongings should be distributed after her death.
The testatrix left her estate to her children in her will.
Testatrix is formed from "testari" (meaning to witness or testify) and the suffix "-trix" (a feminine agent suffix in Latin). The word refers to a female who makes a will or testament, thus acting as a witness to her own legal declarations.
Think of a woman who 'testifies' ('testari') in her will — that's why 'testatrix' refers to a female who creates a will.