His resistance to change is deep-seated, stemming from years of habit.

deep-seated 是由形容词 deep（深的）和动词 seat（使坐下、安置）的过去分词 seated 组成的复合形容词。deep 表示"深的、深刻的"，seated 表示"固定的、确立的"。deep-seated 合起来表示"根深蒂固的、深植于内心的"，形容某种态度或习惯非常牢固，难以改变。