As a familiar form of John, one of the most common of English masculine names, Jack was early applied to any male representative of the common people, especially to a serving man, a laborer, sailor, odd-jobs man, etc.
Thus came such names as steeplejack, jack-tar, etc.
Through association of the name with common labor it became attached also to mechanisms or devices that might be substituted for common labor.
Hence such terms as jackscrew (now usually contracted to jack and operated often by means other than a screw), bootjack, jackhammer, etc.
Or again, the name was applied to animals, birds, and plants, either because of their common occurrence, their usefulness, or their smallness.
Hence such names as jackass, jackrabbit, jackdaw, jack oak, jack pine, jack plane, and so on.
The list of applications is by no means exhausted though most of them fall within the categories mentioned.