We don't have synonyms for crab apple yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are crabapple, crab-apple and crab apples.
(n) :
small sour apple; suitable for preserving; any of numerous varieties of crab apples cultivated for their small acid (usually bright red) fruit used for preserving or as ornamentals for their blossoms
(n) :
small sour apple; suitable for preserving; any of numerous varieties of crab apples cultivated for their small acid (usually bright red) fruit used for preserving or as ornamentals for their blossoms