床上功夫（好或差）
If someone is good/bad in the sack, they are sexually skilled/not sexually skilled.
Isabel used to say she was only with him because he was so good in the sack.
He's a really nice guy and great in the sack.
She's a nice girl but she's not much in the sack.
He's really funny and she's not bad in the sack either.
See also: sack noun (BED)