But that all changed 38 years ago. Forced out of hiding to take over her elderly parents' grocery store in Surin province, Ms Duangjai showed Samrong sub-district neighbours and customers her secret: arms so large she could barely lift them. And on Sept 25, she'll show them to the world.
Ms Duangjai, now 58, suffers from a rare form of macrodystrophia lipomatosa, a gigantism disorder that typically results in one or two abnormally large fingers. In her case, one of just seven known to doctors, it swelled her both arms from the shoulders down.
Her right arm weights 13 kilogrammes and her left 10. Her oversized hands make it nearly impossible to dress, to eat, or to serve snacks and drinks at the food cart she operated at a recent sports day at Ban Samrong School. In a reverse of the usual macrodystrophia condition, her index and pinky fingers remain normal-sized. It is with those that she picks up customers' goods and wields a spoon to eat.
"Both arms are so heavy and painful. It really hurts when I stand and work. I must sit down and place my hands on a table to regain my strength," Ms Duangjai said.
So long a hermit, Ms Duangjai, now lives in Ban Krathom and runs a snack stand at Ban Samrong School where her sister's children go. And in three weeks, she'll be featured on Body Bizarre, a globally syndicated television show from American network The Learning Channel. TLC's early media previews of the show already have put her face - and giant arms - in newspapers around the world.
While Ms Duangjai was raised in a rural area where her condition, normally diagnosed before birth, went undetected, she has in recent years seen a number of doctors and undergone surgeries at Siriraj and Chulalongkorn hospitals in Bangkok. But the condition is so rare, there's no cure yet, only experimental treatments.
One operation was conventional liposuction where 700 grammes of fat and tissue were sucked out of her arms. It proved an excruciating - and ultimately futile - ordeal. The fibrous tissue came back.
Ms Duangjai previously received aid from the Princess Mother's Medical Volunteer Foundation, but she dropped out when her mother fell ill and then recently died. It's now been six years since she has undergone any treatment.
Hope emerged, however, last October when a Japanese film producer and a Thai citizen flew her to Kiatsato University in Japan where her condition finally was diagnosed properly. Ms Duangjai underwent a magnetic-resonance imaging scan, which showed the vast deposits of fat stretching across her arms and pressing on nerves, creating her persistent pain.
Ms Dunagai said her Japanese doctors told her they could remove the fat, but have not contacted her again to arrange for treatment. Her younger sister, Jankaew Samaksamarn, added that the doctors claimed they had treated six patients with enlarged limbs and, if she waited, she could become the seventh.
However, plastic surgeon Dr Eiju Uchinuma is quoted in the TLC documentary as saying her disease is incurable.