treacherous; stealthy; sly
When I heard that he had invited my sister to his house, I thought it sounded like an insidious trick.
in (into, in, on, upon)#sedere (to sit) (Latin)
insidious= inside+ hideous. think of something terrible, like a disease, hiding inside of the body. Relate it with inside as in "something inside or hidden ". Insidous Remember the sith lord-darth sidious. He was a vile treacherous person. Thus insidious = sly, treacherous