by inches

Also, inch by inch. Gradually, bit by bit, as in We found ourselves in rush hour traffic, moving by inches. Shakespeare used this term in Coriolanus (5:4): "They'll give him death by inches." Despite the increasing use of metric measurements, it survives, often as an exaggeration of the actual circumstance. The phrase to inch along, first recorded in 1812, means "to move bit by bit," as in There was a long line at the theater, just inching along.

The adverb has one meaning:

Meaning #1: by a short distance
  Synonyms: little by little, by small degrees