Flat-footed and Off guard

Flat-footed adjective - With feet flat on the ground; not tiptoe.

Off guard and flat-footed are semantically related In some cases you can use "Off guard" instead the phrase "Flat-footed" as an adjective or an adverb.

Off guard adjective - Not prepared or vigilant.

Flat-footed and off guard are semantically related in unprepared topic. You can use "Flat-footed" instead the phrase "Off guard" as an adjective or an adverb, if it concerns topics such as when not paying attention.