The cake was sickeningly sweet, I couldn't stand more than one bite.

sickeningly 是由动词 sicken（使恶心）加上副词后缀 -ingly 构成的副词。sicken 本身由名词 sick（恶心的）加上动词后缀 -en（使变得...）组成。 sickeningly 字面意思是"令人恶心地"，在这里用来强调甜度过高带来的不适感，可以理解为"甜得发腻"、"甜得令人反胃"。