I keep seeing that abbreviation on dict.cc and other dictionaries, but I can't find what it is short for.
Ugs. == umgangssprachlich see also https://dict.leo.org/ende/?lang=de&search=umgangssprachlich
This abbreviation is one of the many metalinguistic signs and symbols used in dictionaries and linguistic works of reference in order to save precious space and to clear the layout.
In printed works, the abbreviations are usually included in the introductory part of a dictionary, i.e. on the first few pages. In online resources, you have to look for links labeled "Abkürzungen", "Abk.", "Hilfe", "Help", "Shortcuts", and the like, but you may also access the abbrevations list through a site search. A search engine query might be most promising if you include "German" or "Deutsch" in the search string, as it was already pointed out in a comment.