A slight outgrowth of hard skin in the thumb
I’m not really sure but I have a Similar thing but I had it as a result of writing for long times 😊
I, too, have developed this from writing. I have it on my knuckle just under the nail on my right middle finger.
Maybe scleroderma or sclerodactyly
Possibly just a callus
No need to worry about it keep smiling 😊