Some sort of strange 4chan meme, usually on said site's /r9k/ board.
Tendies are basically chicken strips or chicken fillets, which I'm told are sometimes called chicken tenders in North America. You take a strip of chicken, breadcrumb it, and fry it or bake it, and that's a tendie. In about 2015, tendies began to appear in posts on 4chan as the centrepiece of a particular genre of story. Usually they're something like this:
The protagonist and narrator of the story is an overaged autistic manchild. He has no job, spends all day at online gaming or the internets, has no inclination to get a job, is colossally fat, and lives with his long-suffering mother. He also has zero or possibly even negative personal hygiene skills and would do anything for his tendies, up to and including bullying his mother. On the subject of personal hygiene, reference is often made to him wearing an adult diaper so he doesn't have to get up to use the bog and miss valuable online gaming or animé time, and reference is also made to (look away now if easily squicked) "pissjugs" and "shitjugs". Sometimes he has a fedora and sometimes a waifu pillow, the latter of which is very crusty. If denied tendies he will act out, usually going "REEEEEEEEEEEEEE" in so doing. A lot of the time he talks in rhyming couplets as well. Just because.
His mother is single, sometimes with a "new daddy," and is at her wit's end to do anything about the protagonist. Weak-willed, her attempts to impose order by requiring the protagonist to obtain GBP or "Good Boy Points" with which to purchase his tendies are subverted, inverted, or just plain ignored by the protagonist. She is often portrayed as the enemy, because she won't give tendies, and anon's conduct often drives her to drinking "adult grape juice" or similar.
Authority figures appear usually as obstacles to the obtaining of tendies. The protagonist deals with them by squashing them under his enormous body weight or flinging his faeces at them.
There is never a point to the story or anything. They fall into a pattern of "anon wants tendies, insufficient GBP, tantrum involving bodily fluids until tendies are obtained." Sometimes the protagonist is literally an overgrown infant and has a cot or a high chair.
Some of the stories get extremely surreal, or insist on subverting the whole premise, like the one where anon decides to earn a billion Good Boy Points by stopping being a basement dweller and getting ahead in life and growing up, only to, once he has obtained that lofty tally, proceeding to literally shit on the floor and scream about how he's been "a good boy for mummy." There was also an incident where a 4chan user trolled Mumsnet by posting the premise of the tendies story to their parenting board from the point of view of anon's long suffering mother. Though to be fair that was seen through fairly swiftly.
The meme seems to be some sort of self referential leaning into 4chan's reputation as a hive of autistic basement dwellers. You don't really see this sort of thing posted much nowadays but various trappings of the tendies stories, especially the pissjugs, have become commonly associated with basement dwellers of all stripes on social media. As is "REEEEEEEE" as a caricature of a social media user or other person having a tantrum about something.
(IRON NODER 2023 #17)