(one) could be (someone's) (parent/grandparent)

One is old enough to be someone else's parent or grandparent. Used in reference to two people who are or may become romantic or sexual partners, especially to indicate that the speaker thinks such a pairing would be inappropriate. He's going out with Mary from the legal department? But she could be his mother! I don't feel comfortable with this. I could be your grandfather, for heaven's sake! I highly doubt Ron and Elise are going out. For one thing, he could be her father. For another, they're always squabbling with each other.

(one) is old enough to be (someone's) (parent/grandparent)

One is old enough to be someone else's parent or grandparent. Used in reference to two people who are or may become romantic or sexual partners, especially to indicate that the speaker thinks such a pairing would be inappropriate. He's going out with Mary from the legal department? But she's old enough to be his mother! I don't feel comfortable with this. I'm old enough to be your grandfather, for heaven's sake! I highly doubt Ron and Elise are going out. For one thing, he's old enough to be her father. For another, they're always squabbling with each other.

grandfather (someone or something)

To exempt certain people or things from new limitations or restrictions, thus allowing them to continue doing or benefiting from something as they did before. This can be done through the use of a "grandfather clause." If they change the pension plan, they better grandfather us! Helmets become mandatory in the NHL in 1979, but the league grandfathered players already active at that point. Only new hires have to live in the city to work here. The rest of us were grandfathered when they drew up the new contract.

grandfather clause

A clause that exempts certain people or businesses from new limitations or restrictions, thus allowing them to continue doing or benefiting from something as they did before. Originally referred to a clause added to the constitutions of some Southern US states that exempted people with relatives that had voted before 1867 from strict new voting requirements, in effect disproportionately limiting the ability of African Americans to vote. No, I still get to pay the reduced rate, thanks to a grandfather clause in my contract. If there are any changes to the pension plan, they better be accompanied by a grandfather clause for existing employees! Don't worry, I'll have the lawyers add in a grandfather clause so that only new hires are affected by this change.

grandfather in

To exempt certain people or businesses from new limitations or restrictions, thus allowing them to continue doing or benefiting from something as they did before. This can be done through the use of a "grandfather clause." A noun or pronoun can be used between "grandfather" and "in." If they change the pension plan, they better grandfather us in! Helmets become mandatory in the NHL in 1979, but the league grandfathered in players active before then. Only new hires have to live in the city to work here. The rest of us were grandfathered in when management drew up the new contract.

(a) grandfather clause

a clause in an agreement that protects certain rights granted in the past even when conditions change in the future. The contract contained a grandfather clause that protected my pension payments against claims such as might arise from a future lawsuit.

grandfather someone or something in

to protect someone or a right through the use of a grandfather clause. My payments were grandfathered in years ago.