1
general::
Immediately beside someone, close by, as in The apprentice was constantly at the master's elbow. Why this idiom focuses on the elbow rather than the arm, shoulder, or some other body part is not known. Moreover, it can mean either that someone is so nearby as to constitute a nuisance or in order to readily provide assistance. Either can be meant in the example above. [Mid-1500s]
American Heritage Idioms