Jeff Psychology is the principle that whatever motivation or intention you imagine another person has is coming from you.
The term closely parallels psychological projection, the defense mechanism first described by Sigmund Freud, in which a person attributes their own feelings or motives to someone else. Where projection names a pattern, Jeff Psychology works as a rule of thumb: before acting on a reading of someone else's intent, look first at where that reading comes from.
The principle applies most directly to working relationships, where assumptions about a colleague's motives can shape a conversation before it begins.