dulcis.

Most likely akin to Ancient Greek γλυκύς (glukús, “sweet”), both traditionally derived from a tentative Proto-Indo-European *dl̥kús (“sweet”), although the fall of -u- in the Latin form would be left unexplained, and the Greek form is problematic as well. Given that Latin and Greek are linguistically quite distant while being very close geographically, it is likely the word does not go back to PIE as it may have been borrowed from a common unknown source, possibly related to lac (“milk”). Compare typologically Persian شیرین (šīrēn, “sweet”) from شیر (šīr, “milk”) and Proto-Turkic *sǖčig (“sweet”) from *sǖt (“milk”).