Hashem Abedi, 22, is accused of plotting with Salman Abedi, who blew himself up with homemade explosives, killing 22 people and injuring many more after an Ariana Grande concert on May 22, 2017.
Among those killed were Sorrell Leczkowski, 14, from Adel, Leeds; Courtney Boyle, 19, a Leeds Beckett University student from Gateshead; Kelly Brewster, 32, from Sheffield; Wendy Fawell, 50, from Otley; and Angelika and Marcin Klis, a couple from York.
It is alleged the brothers had become radicalised and worked together to source materials to build the deadly device.
The Old Bailey heard how Hashem Abedi hoarded old cooking oil cans and skewers from the takeaway where he worked which he allegedly used to make prototype bomb parts.
A friend and former colleague also described how he asked him to buy a large amount of sulphuric acid, a key component in TATP explosives, claiming it was for a generator in Libya.
The witness, who cannot be identified, said he found out it could only be purchased in small quantities and they never spoke of it again.
Cross-examining, Stephen Kamlish QC said: "The type of character Hashem was, you described him as clumsy, a little stupid and like Goofy, the cartoon character, the cartoon dog who gets things wrong."
The witness agreed that Hashem was "just like a normal dozy" youngster and told the court they used to smoke weed together in a park and his "hot box" car.
Making "very big spliffs" was Hashem Abedi's "speciality", the witness added.
Jurors heard Hashem also took other drugs, including tramadol, MDMA (ecstasy), and alcohol - to the disapproval of Salman Abedi, who he referred to as "Big Brother".
Mr Kamlish suggested that Hashem Abedi once had told his friend: "I cannot go home, I'm too messed up."
The defence lawyer said: "The reason was his brother Salman disapproved of all this kind of going on, and on this occasion you went to a restaurant, Archie's, to sober him up and make him more presentable and you thought his brother would not notice.
"He was scared of his brother in the time you knew him. He never wanted Salman to know that he smoked weed or did anything naughty."
Mr Kamlish went on to suggest that Hashem Abedi complained that his brother made him go to the mosque and "follow his religion".
The witness told jurors: "He just wanted to chill, yeah."
Mr Kamlish said Salman Abedi continued to be "on his back" and Hashem did not want to go home because he was "scared".
The lawyer said: "Do you remember he hit him, smacked him?"
The witness replied: "Yeah, he said sometimes."
The jury was told how a group of friends had given Hashem Abedi "birthday beats" for his birthday in April 2017.
Mr Kamlish described it as like a "fake beating" or college fraternity prank.
He said: "On that occasion he was so worried Salman would see him looking like he was taking drugs, he asked you where he could buy eye drops in the middle of the night."
The court also heard that during fighting between youngsters in 2015 and 2016, Hashem had been hit in the face with a hammer.
The witness got to know Hashem Abedi through a takeaway in Greater Manchester where they both worked. Hashem was paid £5 an hour cash in hand as a delivery driver.
His former boss said in a statement read to court: "I could not rely on him to turn up for his shifts. Hashem was religious but in my opinion he had the wrong idea of Islam.
"He would smoke weed and did not appear honest. I heard him ask other members of staff if he could take cash orders. He would tell customers he had no change so he could keep it for himself.
"Hashem asked me if he could take away the empty metal containers from the shop so he could sell them as scrap metal. I agreed to this as it meant I would have less rubbish at the shop.
"Hashem would take any metal tins including pasta sauce tins, jalapeno tins and cooking oil tins. These brands of cooking oil are sold in large metal tins. We would use approximately one tin of cooking oil a week.
"I believe he would take empty containers from other businesses. I don't think he asked them for permission to do this. I don't know if he sold any of the tins."
The former takeaway shop owner, who also cannot be identified, said that other staff had told him Abedi's front garden "looked like a pizza shop with sauce tins lying around".
Hashem Abedi has denied 22 counts of murder, attempted murder and plotting with his brother to cause explosions.