Keith Urban calls his wife Nicole Kidman a ''maniac in the bed'' on his song 'Gemini'.

The country music star - who has daughters Faith, eight, and 10-year-old Sunday with the 'Big Little Lies' star - has confessed that his track, which features the lyric, ''She's a maniac in the bed, But a brainiac in her head'', is about his spouse.

Speaking to The I newspaper about the song on his recent LP 'Graffiti', he spilled: ''The song is actually about Nicole - and she loves it.''

Nicole is a Gemini, and the 51-year-old musician explained that it was his response to his co-writer Julia Michaels asking him to ''describe'' the Australian actress - who also has children Isabella, 26, and Connor, 24, with ex-husband Tom Cruise.

The full verse is: ''She's a maniac in the bed/But a brainiac in her head/And I know that everybody knows/Ooh, that she's both.''

He added: ''It's a fun song. My co-writer Julia Michaels asked me to describe Nicole and that's what came out. 'She is Gemini, but she's not a contradiction. She can roll with things.'''

In 2009, Nicole admitted in an interview with GQ Magazine the she loved experimenting with ''strange sexual fetish stuff''.

The 'Lion' actress - who divorced Tom Cruise in 2001 following a 10-year union - confessed that she has enjoyed exploring with the different aspects of love, sex and relationships over the years.

She explained: ''My life has been about exploring types of love. I've explored obsession, I've explored strange sexual fetish stuff, I've explored the mundane aspect of marriage and monogamy. But I'm still on that journey.''

Meanwhile, Nicole recently admitted her biggest priority in life is to keep her marriage strong.

She said: ''I feel loved and feel absolutely that I'll give back in exactly the same way because, for me, our relationship making it is more important than anything.''