"either of two round projections of a cannon," 1620s, from French trognon "core of fruit, stump, tree trunk," from Old French troignon (14c.), probably from Latin truncus (see trunk (n.1)).
Entries linking to trunnion
trunk n.1
mid-15c., "box, case," from Old French tronc "alms box in a church," also "trunk of a tree, trunk of the human body, wooden block" (12c.), from Latin truncus "trunk of a tree, trunk of the body," of uncertain origin, probably originally "mutilated, cut off," and perhaps from PIE root *tere- (2) "cross over, pass through, overcome."
The meaning "box, case" is likely to be from the notion of the body as the "case" of the organs. English acquired the "main stem of a tree" and "torso of the body" senses from Old French in late 15c. The sense of "luggage compartment of a motor vehicle" is from 1930. Railroad trunk line is attested from 1843; telephone version is from 1889.