to make common: to render or treat something as ritually unclean, stripping it of clean or holy standing; to defile
This verb is built on the adjective κοινός, "common." In ordinary Greek "common" simply meant shared or everyday, but in Jewish religious life the everyday stood opposed to the holy: what was common was not fit to come near God. So "to make common" came to mean "to defile," to pull a person or place out of the clean, holy category and into the profane.
Two shades run through its New Testament uses. In the Gospels Jesus turns the idea inward, arguing that food entering the body does not defile a person; what defiles comes out of the heart. In Peter's rooftop vision the sense is declarative: God tells him not to call common what God has made clean, so the word means to label something unclean rather than to actually pollute it.
Across both shades the concern is ritual status, not physical dirt. To defile in this sense is to treat as ordinary and off-limits what should be counted clean or holy, whether by an act, by a false verdict, or by breaking a boundary God set.