bit on the side

東野圭吾の小説 "New Comer" を読んでいます。

'Ah...' gurgles Shuhei, staring at Taiji in surprise.
'The wife said something about a detective from Nihonbashi Precinct giving you grief about those cakes. There's no need to worry. They've nothing to do with the murder.'
'I wasn't worried...'
'No need to playact with me, kid. I know what you were thinking: that the murdered woman was my bit on the side?'
Taiji held up his pinky finger in the Japanese sign for 'girlfriend.'

小指を立てて「彼女」を意味するのは日本だけの仕草であることが上記引用文の最後で分かります。一応 "bit on the side"　を辞書で確認します。

・Collins Dictionary: informal: an extramarital affair: I'm sure that's what every man looking for a bit on the side says.
・Cambridge English Dictionary: a sexual relationship with someone who is not married to you, or the person you have the relationship with: We've thought for a while that he was having a bit on the side.
・Urban Dictionary: Someone who you're casually dating/sleeping with on the side of a serious relationship or marriage.: Lisa found out about her husband's bit on the side last week.