They were secretly playing footsie under the table during their coffee date.

footsie 是名词，来自 foot（脚）加上后缀 -sie（表示亲昵或小的事物），指"用脚在桌下轻轻触碰对方"的调情游戏。这种游戏通常是情侣间隐秘的亲昵行为。