Uncertain. Explanations include:
* From a formation equivalent to Proto-Indo-European *(s)kolh₂_/₃-trom, from the root *(s)kelH- (“to cut”).
* From the root *(s)ker- (“to shear, cut off”) to a preform *kor-tro- which has undergone dissimilation */rtr/ > /ltr/.
Both of the above etymologies assume a change in the suffix *-trom (and in gender), which otherwise would yield Latin *-trum.
Compare Sanskrit कर्त्तृ (karttṛ), Ashkun kāṭi, Kamkata-viri kaṭo, kṭo, Prasuni iti, Tregami kāṭäy, Waigali kaṭä, from Proto-Indo-European *(s)kért-tōr.