The suffix continues both Proto-Indo-European *-tós and *-tus. A distinguishing feature between the two is that lemmas formed with the u-stem suffix, later were extended with *-kъ, e.g. Serbo-Croatian podátak(“datum”) < *datъ(“load, something given”).
Suffix
*-tъm
Deverbal from mono-syllabic simple stems, forms passive participles