bit by bit in WordNet English dictionary

1. in a gradual manner
   "the snake moved gradually toward its victim"
   gradually  bit by bit  step by step  gradually  bit by bit  step by step 
   
2. a little bit at a time
   "the research structure has developed piecemeal"
   piecemeal  little by little  bit by bit  in stages  piecemeal  little by little  bit by bit  in stages 
   

bit by bit in English Wiktionary