crab apple 
noun (also crab)
1A small sour apple.If you buy a crab apple today, it won't go sour on you....
- If they are very lucky there may be some fruit, wild plums or a crab apple.
- He pulled out his slingshot and loaded it with a crab apple, and took aim at Parsons' broad back.
2 (also crab tree or crab-apple tree) The small tree that bears the crab apple.
- Genus Malus, family Rosaceae: several species and hybrids, in particular the wild Eurasian crab apple (M. sylvestris).
- She had only to stand in the orchard, to put her hand on a little crab tree and look up at the apples, to make you feel the goodness of planting.
- Tree peonies, magnolias, pagoda dogwood and flowering crab trees accompany roses.
Origin
Late Middle English: crab perhaps an alteration (influenced by crab1 or crabbed) of Scots and northern English scrab, in the same sense, probably of Scandinavian origin.