He’s dead Jim is a phrase used in an episode of Star Trek, Tumblr user, Salmon-Stan made a post where he showed it saying multiple puns likes “he’s Thread Jim” and “he’s bread Jim” etc. This gained large popularity on Tumblr. Tumblr user, Salmon-Stan is likely the one who made it first, there it first came popular, and gained more popularity when user, Motomenorahkent reblogged it.