A conker was a children's toy made from a horse chestnut seed and used to play a game of the same name.

When the Second Doctor visited the Panjistri homeworld, he had a pair of conkers in his pocket. (PROSE: Timewyrm: Apocalypse ^[+]Nigel Robinson, Virgin New Adventures (Virgin Books, 1991).)

The Second Doctor explained the concept of a space elevator to a confused Jamie McCrimmon by using a conker on a string as a demonstration. (AUDIO: The Great Space Elevator ^[+]Jonathan Morris, The Companion Chronicles (Big Finish Productions, 2008).)

The Third Doctor had Venusian conkers. (AUDIO: Ghost in the Machine ^[+]Jonathan Morris, The Companion Chronicles (Big Finish Productions, 2013).)

The game was introduced by the Seventh Doctor in 8th century Britain. (COMIC: Doctor Conkerer! ^[+]Ian Rimmer, DWM Comics (Marvel Comics, 1990).)

On Olleril, the Seventh Doctor used a conker on a string to hypnotise the Robert Clifton Celebroid. (PROSE: Tragedy Day ^[+]Gareth Roberts, Virgin New Adventures (Virgin Books, 1994).)

The Eighth Doctor carried a conker in his pocket. (PROSE: Kursaal ^[+]Peter Anghelides, BBC Eighth Doctor Adventures (BBC Books, 1998).)

The Tenth Doctor carried a conker in his pocket. (PROSE: The Price of Paradise ^[+]Colin Brake, BBC New Series Adventures (BBC Books, 2006).)

During a fight with Cybermen, the Tenth Doctor wished they would agree on settling it with a conkers fight. (COMIC: The Power of the Cybermen ^[+]Steve Cook, DWBIT comic stories (GE Fabbri Ltd, 2006).)

The Tenth Doctor compared Bannakaffalatta to a talking conker. (TV: Voyage of the Damned ^[+]Russell T Davies, Doctor Who Christmas Special 2007 (BBC One, 2007).)