right-hand man

Also, right-hand woman. A trusted helper, as in Give it to Jill, she's my right-hand man. Based on the idea that in most people the right hand is the stronger of the two, this idiom today usually disregards gender, as in the example. However, in the the 17th and 18th centuries it also meant a soldier in a post of command on the right side of a cavalry unit, and then always denoted a man. [c. 1800]

The noun has one meaning:

Meaning #1: the most helpful assistant
  Synonyms: chief assistant, man Friday