rabo verde 

An older man who likes to court and date younger girls, whether corresponded or not. Only used in extreme cases (a 45 man courting a 18 year old girl would be appliable). It is most exclusively used with men and in the phrase: 'viejo rabo verde' (viejo meaning old man) No equivalent expression for women.

¡¡¡Ya déjala en paz, viejo rabo verde, que podría ser tu hija!!!!
(leave her alone, you viejo rabo verde, she could be your daughter!)

rabo verde by thekingofhell July 24, 2004

rabo verde 

also called "cebolla", Spanish for Onion, because of their white hairs ( old men) and their green butt ( "rabo verde")

"a ese viejo rabo verde le dicen la cebolla" meaning: that old man is also called la cebolla

rabo verde by Zerimar June 1, 2005