1) Toast was previously part of one of the groups of raiders running loose around the Citadel. Maybe even one of the Rock Riders or Buzzards, which would explain her familiarity with bullets and guns.
She was either kidnapped from them, or given to Joe by them as a sortof hostage, so that Rigs going between the Citadel, Bullet Farm, and Gas Town would not be attacked so long as they stayed on the road and within their territory. She was given to Joe as a symbol of their uneasy cease-fire agreement.
2) Toast has scars all over her shoulders, chest, and arms, both from wearing scrappy makeshift Buzzard armor that left her with small cuts and friction burns, but also from being shot at or stabbed while riding with the Buzzards. This is why her outfit is the only one of the wives that has sleeves and covered shoulders. Immortan Joe didn’t want to be reminded of her rough past, which he finds unsuitable for a wife.
3) Toast used to have long hair like the rest of the wives ( when she was a Buzzard, she could wear it braided up under a helmet, for extra protection against sharp blows, or worn down to protect her from the sun). Her hair was very long and thick, and wavy or curly. Joe always said her hair was the best thing about her. So, of course, every time she got her hands on something sharp, or, later, on something that Could Be Sharpened, the first thing she’d do was slice off huge chunks of her hair. Eventually Joe had someone cut it all short because he was tired of fighting her on this, but made her wear a hood. Again, to cover up the physical reminder of her rebellion, so he wouldn’t have to see it.