Origin and history of trunks

[main part of something, as distinguished from its appendages] mid-15c., "box, case," from Anglo-French trunke, tronke, variants of Old French tronc "alms box in a church," also "stem of a tree from which the branches spring; trunk of the human body; wooden block" (12c.), also from Medieval Latin truncus.

Both are from Latin truncus "trunk of a tree; trunk of the body; wooden block," a word of uncertain origin, probably originally "mutilated, cut off." 

The post-classical development of the meaning "box, case with a lid or top" is likely to be from the notion of the body as the "case" of the organs. Especially a case for conveying clothes, etc. on a journey. The sense of "luggage compartment of a motor vehicle" is from 1930 (compare boot (n.1)). 

English acquired the "woody main stem of a tree" and "torso of a human or animal body" senses from Old French in late 15c.

Extended to blood vessels, etc.; the railroad trunk line is attested by 1843; the phrase in reference to telephone lines is by 1889. Trunk-hose (1630s) apparently so called in reference to covering the body, as distinguished from the limbs.