Green’s Dictionary of Slang

(US) one who works on a car or cars with minimal equipment and other than in a purpose-built environment.

1973		J. Wambaugh Onion Field 331: ‘You used to say you were a pretty good shade-tree mechanic.’ ‘I can’t fix anything,’ said Karl.
1981		A. King q. in McKee & Chisenhall Beale Black & Blue 252: ‘[T]his friend was a little shadetree mechanic; I would help him around’.