Green’s Dictionary of Slang

(US drugs) in pl., punctures and scar tissue from injections that accumulate along the veins of a regular heroin addict.

1953

W. Brown Monkey On My Back (1954) 132: I knew he was not ‘main-lining’ it through his wrists or arms as I had asked Mrs. Maldano to watch for any fresh ‘hitches’. [Ibid.] 134: As I had suspected, there was a row of ‘mainline hitches’ along the soft inner flesh of his left thigh.