I am a layman when it comes to medicine; I don't understand anything.
Tips
memory
Literally 'a Han person outside the door', someone who stands outside the door of knowledge and hasn't entered the field. 门 = door, 外 = outside, 汉 = man/person.
usage
门外汉 is informal and mildly self-deprecating. Use it to humbly admit you know little about a topic, e.g. 我是个门外汉 (I'm just a layman).
