the.com/cash cow

means A product, business, or asset that reliably generates steady profit with little ongoing effort or investment required to sustain it.

from Straightforward agricultural metaphor from mid-20th-century American business jargon: a dairy cow doesn't need to be slaughtered or replaced to produce value — you just milk it, day after day, for a steady, low-effort yield. Business writers borrowed the image directly, and it stuck because the comparison needs no explanation.