flat是平的意思。back是后背的意思。flat on his back就是病得起不来床，卧病在床，（经济市场等）不景气的意思。

【例句】
She’s been flat on her hack for over a week now.
她卧病在床一个多星期了。

The market was flat on itsback.
当时市场十分不景气。