the.com/shade tree

means A tree grown or valued mainly for the cool, leafy cover it casts rather than for fruit, timber, or any other product — shelter as its whole job description.

from Simply compound and literal: 'shade' (from Old English sceadu, meaning shadow or shelter from light) plus 'tree,' joined in English once people started explicitly planting trees for the comfort of their shadow rather than for wood or food. The phrase picked up a second, metaphorical life in American vernacular — 'shade tree mechanic' for a self-taught fixer who works outdoors under a literal tree rather than in a proper shop — extending 'shade tree' from botany into a byword for the informal, the homegrown, and the unlicensed-but-capable.