Origin and history of trunk

[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.

"long snout of an elephant" or other animal, 1560s, apparently from trunk (n.1), perhaps from confusion with trump (n.2) "trumpet." Early uses of the word are in reference to the appendages' ability to hold water. Slang use in reference to the human nose is by 1700.