"Dude Weed Lmao" is a catchphrase used to impersonate and mock members of the stoner subculture and as a spam and shitposting practice. The catchphrase is also associated with the Canadian actor, filmmaker, and comedian Seth Rogen, known for featuring marijuana-related jokes or scenes on his movies. The phrase can be used as a criticism of some kinds of comedy, under the phrasal template "Dude X Lmao".
The first instance of the catchphrase can be found on a thread submitted on 4chan's /tv/ board on July 15th, 2014, featuring a close-up photo of Seth Rogen's face along with the message "DUDE WEED LMAO", mocking his tendency to feature stoner humor in his films. The same thread also featured posts that mock Rogen's comedy tropes.
During the following weeks, a series of posts and threads with Seth Rogen's picture combined with the catchphrase were created on /tv/, mostly due to the spread of the trailer for his film The Interview, and the posts spread to other boards. At some point the catchphrase also started being used as a way to refer to marijuana enthusiasts or defenders. On September 16th, 2014, a thread asking for the meaning of the catchphrase was submitted on /tv/.