「厚い」(atsui) refers to something having a large dimension from one surface to the opposite, like a thick book or a thick slice of bread. It can also describe a deep friendship. Example: この本はとても厚いので、読むのに時間がかかります。(This book is very thick, so it takes time to read.)

厚みがあること。親しい関係を言うことも。