answer:I think it’s the property of male genitals to be lumpy. In this case: 2. transf. A piece or lump of anything; a CHUNK. [Chunk may have originated under the joint influence of chuck and junk.] 1726 G. ROBERTS 4 Years Voy. 155, I..gave to each of them a short Junk of Pipe. 1764 GRAINGER Sugar Cane 1. Note 41 The stem is knotty, and, being cut into small junks and planted, young sprouts shoot up from each knob. Ibid. III. 127 The Cane..Cut into junks a yard in length. 1833 M. SCOTT Tom Cringle i. (1859) 8 A large knot in his cheek from a junk of tobacco therein stowed. 1843 MRS. CARLYLE Lett. I. 270 [He] snatched up a large pound-cake, cut it into junks. 1876 M. E. BRADDON J. Haggard’s Dau. xxiii. 243 The huge junk of single Gloucester. (Oxford English Dictionary)