Wiktionary
brown thumb
n. (context idiomatic English) Lack of skill at growing plants; something possessed by a poor gardener.
Usage examples of "brown thumb".
She was sleeping in a foetal bundle: white cotton nightie, kneecaps for breasts, her little brown thumb planted tritely in her mouth.
Smoothing the pink underside of his brown thumb down her pert chin, Magnus asked playfully, 'What's you thinking about then ?