It means "unwelcoming/unsociable" (constructed from on (negation prefix) + gezellig)

And, furthermore, "gezellig" means 'having an inviting and friendly ambiance; cozy, nice, pleasant, sociable'.

…Furthermore, "gezellig" is itself a compound ("gezel" + "ig" (adjective-forming suffix)), gezel means companion, and archaically, journeyman.

Most literally, "ongezellig" would be translated as "uncompanionlike".