The guy suing Nirvana for featuring his naked baby body on their famous 1991 album cover seems to have forgotten to come as he was to court -- so now his lawsuit's been thrown out.

Spencer Elden's lawsuit against the band -- not to mention Kurt Cobain's estate -- has been dismissed, for now, with the judge noting dude and his lawyers failed to respond to a motion to toss the entire thing filed by the defendants ... a deadline for which had been set on Dec. 30.

That day apparently came and went without any word from Elden and co. -- so, the judge granted Nirvana's wish of nixing the whole suit ... at least for the time being. Elden now has until Jan. 13 to submit an amended complaint if he wants to keep pressing the issue.

As for what was in that motion that Elden never came to the table for ... it was full of arguments from Nirvana's attorneys that said not only did this guy wait too long to file his child pornography claims -- but also that Elden himself had apparently leaned into his Nirvana fame on several occasions over the years.

As we reported ... Elden claimed he was violated when his then 4-month-old naked self was plastered on the "Nevermind" album after alleged promises it would be censored ... saying it caused him to suffer lifelong damages. He was asking for a pretty penny to make himself whole.

Elden's legal team refused to comment on the latest update or any future plans. A strange U-turn, no doubt -- almost as if he sat down and thought ... never mind!